fix(dashboard): make optimistic↔canonical bubble convergence testable behind a pure seam

[sethvoltz]

What & why

ChatState reconciled three overlapping state layers (optimistic / streaming / canonical) into the rendered bubble list as a render-time $derived filter plus convergence side effects scattered across four SSE handlers and applyZeroBlocks. That merge — the exact spot the project's state-machine testing discipline points at (both optimistic↔canonical interleavings; reload-mid-state) — had no isolated test surface: every case required standing up the whole store (runes, Zero wiring, SSE plumbing, transport injection), so the bug class lived in code nothing could unit-test.

This lifts the merge into one pure, clock-free, rune-free module — services/dashboard/src/lib/stores/bubble-convergence.ts — with a narrow interface. ChatState keeps transport, focus, pagination, timers, and all reactivity; it now calls the convergence core across a real seam.

Shape

The core owns the shared types (ChatMessage, OverlayKey/overlayKey) and the previously in-store pure helpers (parse*, *IdForTurn, filterRowsToCurrentSession, dropSupersededNoResponseSafetyNet, oldestBlockCursor, …) — moved, not imported, so the core imports neither store and the chat↔zero circular import is not reintroduced. ChatState re-exports the public surface for existing consumers.

Pure interface:

• mergeBubbles(legacy, streaming, optimistic, focus) -> ChatMessage[] — entries returned by reference; reads only identity fields (id/agent/sessionId) so a per-delta entry.text += … $state mutation never re-runs #derivedMessages (the hot-path reactivity contract).
• pruneConverged(legacy, streaming, focusAgent) -> {keep, drop}
• reconcileComplete(snapshot, event) -> ReconcilePlan — one dispatcher + 5 unexported per-kind helpers; the shell applies the discriminated plan via one switch.
• reconcileCanceled / userBubbleAlreadyLanded / mergeZeroSnapshot

Behavior

Convergence behavior is preserved — the 228-test chat.test.ts regression suite passes untouched. The fix is closing the test gap and pinning the invariants.

Tests (ship in the same change, per the state-machine testing discipline)

• +28 pure convergence unit tests (bubble-convergence.test.ts) — pinned shapes/counts and by-reference identity, no store/runes/Zero.
• +4 store-level integration tests in chat.test.ts: POST-then-SSE and SSE-then-POST interleavings, reload-mid-state, agent-agnostic multi-agent cancel, and a discriminating perf-subscription contract probe.

The pure extraction also pins several latent convergence invariants that were previously unguarded: the reachedOldest two-writer pagination reset, zeroSeenBlockIds write-back ordering, agent-agnostic cancel, and the merged-vs-raw dedup read surface.

chat.svelte.ts net −1455 lines.

Verification

• chat.test.ts + bubble-convergence.test.ts: 256 passing.
• Full dashboard unit suite: 943 passing / 66 files.
• svelte-check: 0 errors (1 pre-existing unrelated warning).
• prettier --check: clean. Core imports neither store (acyclicity verified).

Type is fix → release-please patch bump.

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