Port the fork UI patch stack to the app overlay: the yellow-chunks grammar (#853)

[aarontrowbridge]

What

Ports the yellow-chunks patch stack to the app overlay — the #853 scope that remains after S2 (posture indicator, #864) and S1 (agent_order, #862) landed.

A scoping discovery shaped the port: the issue's "10 commits" range is 9 content commits + the merge f3aad3cc4. Of the two pre-merge commits, the fork's own merge resolved one away: fe6eac6b1 (section-transition chunking) was NOT carried — the heading-anchored segmentation (which the overlay already has, byte-identical to the yellow-chunks tip) is the live design. dc116cd6c (the base yellow chip) DID land and was missing from the overlay. The port is therefore 8 content commits' worth of design.

The overlay's target files were byte-identical to the fork base (d161eb0c) the stack was written against, so the diffs ride nearly verbatim — re-homed only where the port demands (docket → the ui package beside edit-row.ts; tests → the extension vitest lane).

The grammar (commit 1:1 with the fork's design)

1. The yellow chip (dc116cd6c) — the user's words are the solid yellow chip in both schemes: --accent fill, --accent-ink, per-scheme edge token (--prompt-bubble-edge: ink border on light, transparent on dark); the sticky ghost bubble carries the same tokens.
2. Yellow answers + WAAPI bubble lock (8c91b45fa) — the answered-question's answer renders with the --prompt-bubble-* tokens right-aligned to the user's side; the sticky bubble drops the YOU label; hand-offs merge via a 240ms WAAPI pass (element persists, honors prefers-reduced-motion).
3. The docket (6dac9ce04) — collapsed tool rows carry their evidence: edited-files rows show per-file tokens with summed ±, explored rows show file/pattern/dir tokens with repeat counts, shell rows surface the failure count. Row = summary, dropdown = detail.
4. The slab (1bd3ae7d7) — fenced code inside prose fragments leaves the prose-card grammar: full-bleed across the fragment gutters, quiet layer-02 ground, language label top-right, shell-kind's nested box silenced.
5. Prompt bubble dark inversion (99cb3f93d + 1e2e44b66 net) — on dark the chip inverts into a layer-01 dark box with the FULL-strength yellow border (one step louder than the fragments' --accent-edge hairline). The prose-fragments yellow-edge pass was reverted on the fork the same day — the recording comment lands; fragments stay on the theme hairline in both schemes.
6. Dropdown detail (85d3eaf04) — read/edit rows carry clickable file targets (opens in the editor, full path tooltip); shell rows gain the status dot (running pulse / error / done), the $ prompt, wall duration, exit code when nonzero, one-line output preview.

Tests (the overlay's headless pattern — #848/#859/#862 precedent)

packages/extension/test/yellow_chunks_853.test.ts (18 tests): the full fork test cases for the docket helpers and shellRowDetail, plus CSS-grammar regression guards for the slab / docket slots / yellow answer chip / per-scheme bubble seating / shell command anatomy (the #349 deletion bug class).

Gates

• drift gate PASS (upstream archive sha verified + 617-file overlay/manifest sync); manifest hashes registered for all 8 edited overlay files
• pnpm run typecheck clean; extension vitest: 3106 passed — pre-existing local failures only (terminal ×2 fails on clean main too; cli_gate ×1 is partial-bin residue in the worktree, skips on pristine CI)
• i18n: no new keys (the docket/anatomy keep the fork's hardcoded summary words; aria-labels as on the fork) — parity ×18 untouched
• CI green: all 9 checks (app-bundle-gate, bundle-build-gate, fast, schema-roundtrip, vsix-gate, boot-smoke ×3, app-shelf-boot-proof)
• no engine changes, no fork edits (read-only), no behavior changes beyond the visual grammar

Refs #853

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