diff --git a/CLAUDE.md b/CLAUDE.md
index 42a837c..3b3d3bb 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -105,12 +105,48 @@ Rules:
   `saezuri-illustrations` repo (a flat folder the app auto-downloads per detected
   species); the AvianVisitors cutouts under `public/assets/` remain gitignored dev
   placeholders so the UI is testable locally. Growing that set is ongoing, not a v1 gate.
-## Common commands
+## Code map
+ 
+Where things live, so a change lands in the right place fast.
  
-To be filled in once the scaffold lands. Expected shape:
+- **Collage render path:** `src/pages/CollagePage.tsx` fetches the data and renders
+  `src/collage/Collage.tsx`, which measures the viewport, resolves each species to art,
+  packs the tiles, and maps them to `src/collage/BirdTile.tsx` (one absolutely-positioned
+  `<button><img></button>` per bird). Silhouette hover is arbitrated at the container
+  (`hitTest.ts`), not per tile — the tiles are `pointer-events: none`.
+- **Layout / packer:** `src/collage/layout.ts` — `computeLayout(inputs, vp)` is the
+  deterministic, seeded (`src/lib/prng.ts`) count-driven sizing + silhouette packer
+  (`pack.ts`); reimplemented from AvianVisitors, not copied. Same inputs + viewport ⇒ same
+  layout, so polls and resizes don't churn. `layoutSignature(tiles)` fingerprints the
+  arrangement (`sci`/`n`/`key`, viewport-independent) via `src/lib/hash.ts` (`fnv1a`).
+- **Entrance bloom:** the `gtile-in` keyframes in `src/index.css` (`.gtile.entering`),
+  disabled under `prefers-reduced-motion`. It is a **one-shot CSS mount animation** — it
+  replays only when React remounts a tile, i.e. when the tile `key`
+  `` `${blossomKey}:${sig}:${sci}` `` changes: `blossomKey` is the window preset and `sig`
+  is `layoutSignature`. So the bloom fires on load, window switch, and any in-place update
+  (poll / focus revalidation) that yields a genuinely different layout — not on identical
+  polls or plain resizes. There is no positional transition on `.gtile`; tiles that don't
+  remount just snap.
+- **Data hooks (browser reads static files only):** `src/hooks/useRecentSpecies.ts`
+  (`/snapshot.json`, 12s poll) selects the active window; `src/hooks/useLayoutManifest.ts`
+  (`/layout-manifest.json`, 30s poll) supplies per-species masks/dims/versions. Both use
+  SWR's default `revalidateOnFocus`. The dictionary hooks (`useDictionaryIndex.ts`,
+  `useSpeciesDictionary.ts`) deliberately set `revalidateOnFocus: false`. There is no
+  `SWRConfig` provider. User preferences (`useThemePreference.ts`, `useLanguagePreference.ts`)
+  are per-client `localStorage` under `saezuri:*` keys.
+- **Domain (framework-free, shared with the server):** `src/domain/` — `species.ts`
+  (aggregation + localization), `asset.ts` (`resolveArt`, `imagePath` with the `?v=` hash
+  cache-bust), `snapshot.ts`, `manifest.ts`, `slug.ts`.
+- **Refresh service (the sole BirdNET-Go client):** `src/server/` — holds the SSE stream,
+  gates/aggregates species, and publishes `/snapshot.json`, `/layout-manifest.json`, and
+  the e-ink PNG frames (`render.ts`, reusing `computeLayout`). Run it with `npm run refresh:dev`.
+ 
+## Common commands
  
-- `npm run dev` for the Vite dev server.
-- `npm run build` for the production bundle.
+- `npm run dev` (Vite dev server) / `npm run dev:mock` (`VITE_MOCK=1`, synthesizes species
+  from the local manifest so the collage runs with no backend).
+- `npm test` (vitest), `npm run typecheck` (`tsc --noEmit`), `npm run check` (Biome).
+- `npm run build` for the production bundle; `npm run refresh:dev` runs the refresh service.
 - `docker compose up --build` to run the container against a configured `BIRDNETGO_URL`.
 ## Git hygiene
  
diff --git a/src/collage/Collage.tsx b/src/collage/Collage.tsx
index cab346f..27b7e15 100644
--- a/src/collage/Collage.tsx
+++ b/src/collage/Collage.tsx
@@ -12,7 +12,7 @@ import type { Species } from '../domain/species.ts'
 import { BirdTile } from './BirdTile.tsx'
 import { HoverChip } from './HoverChip.tsx'
 import { hitTest } from './hitTest.ts'
-import { computeLayout, type LayoutInput, type Viewport } from './layout.ts'
+import { computeLayout, type LayoutInput, layoutSignature, type Viewport } from './layout.ts'
 import { decodeMaskCached } from './pack.ts'
 
 interface Props {
@@ -20,9 +20,11 @@ interface Props {
   manifest: LayoutManifest
   /** Bloom tiles in on mount (disable for screenshots). */
   animate?: boolean
-  /** Namespaces the tile keys so a change remounts every tile — used to replay
-   *  the entrance bloom when the whole set turns over (e.g. switching windows),
-   *  while a same-key poll still re-blooms only newly-arrived birds. */
+  /** Namespaces the tile keys so a change remounts every tile and replays the
+   *  entrance bloom. Combined here with a signature of the current layout, so the
+   *  bloom also replays when an in-place update (poll / focus revalidation) yields
+   *  a genuinely different arrangement — not only when the window switches. Pass
+   *  the window preset; it keeps windows in separate key namespaces. */
   blossomKey?: string
   /** Rendered when there are no birds in the window. */
   emptyState?: ReactNode
@@ -101,6 +103,12 @@ export function Collage({ species, manifest, animate = true, blossomKey = '', em
     return computeLayout(inputs, vp)
   }, [species, manifest, vp])
 
+  // Fingerprint the arrangement (species / counts / art slots, not pixel coords)
+  // so the tile keys below change — and the bloom replays — exactly when a poll or
+  // focus revalidation lands a genuinely different layout, and never on a plain
+  // resize or an identical poll. Memoized so hover re-renders don't recompute it.
+  const sig = useMemo(() => layoutSignature(tiles), [tiles])
+
   const fallbackUrl = imagePath(manifest.fallbackKey, manifest.ver?.[manifest.fallbackKey])
   const cx = vp.width / 2
   const cy = vp.height / 2
@@ -137,7 +145,7 @@ export function Collage({ species, manifest, animate = true, blossomKey = '', em
             const dist = Math.hypot(t.x + t.w / 2 - cx, t.y + t.h / 2 - cy)
             return (
               <BirdTile
-                key={`${blossomKey}:${t.sci}`}
+                key={`${blossomKey}:${sig}:${t.sci}`}
                 tile={t}
                 animate={animate}
                 delayMs={Math.min(MAX_BLOOM_DELAY_MS, dist * BLOOM_DELAY_PER_PX)}
diff --git a/src/collage/layout.test.ts b/src/collage/layout.test.ts
index c6b580d..5b50624 100644
--- a/src/collage/layout.test.ts
+++ b/src/collage/layout.test.ts
@@ -1,5 +1,11 @@
 import { describe, expect, it } from 'vitest'
-import { computeLayout, type LayoutInput, tuning } from './layout.ts'
+import {
+  computeLayout,
+  type LaidTile,
+  type LayoutInput,
+  layoutSignature,
+  tuning,
+} from './layout.ts'
 import type { DecodedMask } from './pack.ts'
 
 function solidMask(w: number, h: number): DecodedMask {
@@ -69,3 +75,38 @@ describe('computeLayout', () => {
     expect(first).toEqual(again)
   })
 })
+
+function tile(sci: string, n: number, overrides: Partial<LaidTile> = {}): LaidTile {
+  return { ...input(sci, n), x: 0, y: 0, w: 10, h: 10, parked: false, ...overrides }
+}
+
+describe('layoutSignature', () => {
+  const base = [tile('a', 3), tile('b', 1), tile('c', 5)]
+
+  it('is stable for the same species, counts, and art keys', () => {
+    expect(layoutSignature(base)).toBe(layoutSignature([tile('a', 3), tile('b', 1), tile('c', 5)]))
+  })
+
+  it('ignores tile order', () => {
+    expect(layoutSignature(base)).toBe(layoutSignature([tile('c', 5), tile('a', 3), tile('b', 1)]))
+  })
+
+  it('ignores pixel coordinates, so it holds across a resize', () => {
+    const moved = base.map((t) => ({ ...t, x: t.x + 100, y: t.y + 40, w: t.w * 2, h: t.h * 2 }))
+    expect(layoutSignature(moved)).toBe(layoutSignature(base))
+  })
+
+  it('changes when a detection count changes', () => {
+    expect(layoutSignature([tile('a', 3)])).not.toBe(layoutSignature([tile('a', 4)]))
+  })
+
+  it('changes when a species enters or leaves', () => {
+    expect(layoutSignature(base)).not.toBe(layoutSignature([tile('a', 3), tile('b', 1)]))
+  })
+
+  it('changes when the resolved art key changes (fallback→art, perched→flight)', () => {
+    expect(layoutSignature([tile('a', 3, { key: 'a' })])).not.toBe(
+      layoutSignature([tile('a', 3, { key: 'a-2' })]),
+    )
+  })
+})
diff --git a/src/collage/layout.ts b/src/collage/layout.ts
index 9ac7f76..bb99886 100644
--- a/src/collage/layout.ts
+++ b/src/collage/layout.ts
@@ -1,3 +1,4 @@
+import { fnv1a } from '../lib/hash.ts'
 import { type DecodedMask, isParked, maskPack, type PlaceableTile } from './pack.ts'
 
 // Count-driven sizing + packing, reimplemented from study of AvianVisitors'
@@ -173,3 +174,16 @@ export function computeLayout(inputs: readonly LayoutInput[], vp: Viewport): Lai
     parked: isParked(t),
   }))
 }
+
+/** A short fingerprint of what makes a layout the layout it is: which species are
+ *  present (`sci`), how loud each is (`n`, which drives tile size), and which art
+ *  slot each resolved to (`key`, which sets the mask + aspect and flips on
+ *  fallback→real-art or perched→flight). Sorted so tile order never matters, and
+ *  deliberately blind to pixel coordinates — so it is stable across viewport
+ *  resizes and identical polls, and changes only when the arrangement genuinely
+ *  differs. The collage feeds it into the tile keys so a changed layout remounts
+ *  the tiles and replays the entrance bloom (see Collage.tsx). */
+export function layoutSignature(tiles: readonly LaidTile[]): string {
+  const parts = tiles.map((t) => `${t.sci}:${t.n}:${t.key}`).sort()
+  return fnv1a(parts.join('|')).toString(36)
+}
diff --git a/src/lib/hash.ts b/src/lib/hash.ts
new file mode 100644
index 0000000..3bf212e
--- /dev/null
+++ b/src/lib/hash.ts
@@ -0,0 +1,15 @@
+// FNV-1a, the 32-bit variant. A fast, dependency-free string hash used to fold a
+// long fingerprint down to a short, stable token — the e-ink frame signature and
+// the collage's re-bloom key both fold their per-species strings through this so
+// two identical inputs always collapse to the same value.
+const FNV_OFFSET = 0x811c9dc5
+const FNV_PRIME = 0x01000193
+
+export function fnv1a(str: string): number {
+  let h = FNV_OFFSET
+  for (let i = 0; i < str.length; i++) {
+    h ^= str.charCodeAt(i)
+    h = Math.imul(h, FNV_PRIME)
+  }
+  return h >>> 0
+}
diff --git a/src/server/render.ts b/src/server/render.ts
index ec8066a..5b9027e 100644
--- a/src/server/render.ts
+++ b/src/server/render.ts
@@ -6,6 +6,7 @@ import { decodeMaskCached } from '../collage/pack.ts'
 import { resolveArt, rollFlight } from '../domain/asset.ts'
 import type { LayoutManifest } from '../domain/manifest.ts'
 import type { Species } from '../domain/species.ts'
+import { fnv1a } from '../lib/hash.ts'
 import { createPrng } from '../lib/prng.ts'
 
 // Frame compositor: renders a window's gated species into a fixed-size PNG for
@@ -27,18 +28,6 @@ export interface FrameOptions {
 const SHADOW = { color: 'rgba(26,22,18,0.1)', blur: 6, offsetY: 2 }
 const DEFAULT_AR = 1.4
 
-const FNV_OFFSET = 0x811c9dc5
-const FNV_PRIME = 0x01000193
-
-function fnv1a(str: string): number {
-  let h = FNV_OFFSET
-  for (let i = 0; i < str.length; i++) {
-    h ^= str.charCodeAt(i)
-    h = Math.imul(h, FNV_PRIME)
-  }
-  return h >>> 0
-}
-
 /** Stable seed per species so its pose holds across renders (no e-ink churn),
  *  while ~FLY_PROB of the roster still fly. */
 function seedFor(sci: string): number {