docs(impl): record the hero brief, and why the layout waits on it

IMPL.md

@@ -15,6 +15,56 @@ lines.
 
 ---
 
+## Blocked on an asset: a hero that leaves room for the words
+
+The landing page centres its copy because the artwork is square and symmetric, which
+leaves nowhere clean to set a left-aligned column. The legibility fix (2026-09-20)
+made the copy readable over it, but a veil dense enough to carry text is also dense
+enough to dim the art it covers. The sister sites that read best (periplus, ariadne)
+put the copy left and the art right, on deliberately empty ground.
+
+Replacing the art is what unblocks that layout. Regenerating it needs three files,
+not one:
+
+| file | current | needed | used by |
+|---|---|---|---|
+| `docs/assets/phalanx-hero.png` | 1678x1641 | ~2560x1440 | dark scheme, `extra.css:54` |
+| `docs/assets/phalanx-hero-light.png` | 734x740 | ~2560x1440 | light scheme, `extra.css:29` |
+| `docs/assets/phalanx-og.png` | 1200x630 | 1200x630 | social card, `overrides/main.html:17` |
+
+The aspect ratio is the load-bearing part. `background: ... / contain` on a square
+image letterboxes it in a wide viewport, and a square frame cannot hold an empty
+left third, so a 1:1 source forecloses the layout no matter what is drawn in it.
+
+### Prompt
+
+> Isometric view of a dark field of many small identical compute nodes arranged in
+> loose phalanx formation, each emitting a thin luminous telemetry trace; the traces
+> converge rightward and braid into a single ribbon of light. Deep slate-teal base,
+> emission only in cyan and violet. The left third of the frame falls off to
+> near-black empty space with no detail. Cinematic depth of field, subtle volumetric
+> haze. No shields, no crests, no heraldry, no faces, no symmetry, no text.
+
+For the light variant, the same scene on a pale ground: near-white empty left third,
+the nodes and traces in the same cyan and violet at higher saturation so they hold
+against the paler field.
+
+Two things the prompt is deliberately steering away from. The current art reads as
+heraldry rather than as *many clients, telemetry, aggregation*, which is what the
+system does; and its symmetry is what forces the centred type. The palette stays on
+the existing `--intefl-teal` / `--intefl-purple` tokens so the page does not have to
+be recoloured around a new image.
+
+### When the asset lands
+
+`extra.css` needs the hero switched from `center 48px / contain` to a right-anchored
+`cover`, the flex container from `align-items: center` to `flex-start`, and the copy
+column capped near `min(34rem, 42%)` with a scrim measured from that column's edge
+rather than from a share of the viewport — the mistake that made ariadne's hero
+unreadable, fixed there in ariadne#46.
+
+---
+
 ## Open bugs & findings
 
 ### Dependabot: 5 open alerts are upstream pins in flwr 1.36.0 (2026-09-13)