fix(web): keep file edits saved and correctly highlighted

[rynfar]

The file editor could lose autosave after React effect replay, accept stale worker highlights after an edit, or enable editing before its language grammar was ready. Markdown previews could retain widgets from another file. Keep a fresh save coordinator for each mounted file, retire old callbacks, invalidate stale worker tasks/cache entries and load the selected grammar before editing. Reset Markdown widgets across file, workspace and environment changes.

Adopts approved B8: upstream #9878, #9902, #9947 and #9910. Keeps Pylon’s Pierre 1.3.0-beta.10 dependency, existing patches, 500 ms debounce, confirmed file cache and environment-routed writes. Test-only React renderer matches Pylon’s web React version. Web and desktop share this editor; native mobile uses its own surface.

Validation: 31 focused save lifecycle/coordinator and real-worker highlight/grammar tests pass, plus web typecheck, targeted lint and diff checks. Tests cover StrictMode replay, close/reopen, retired callbacks, 7,000-line source edits and stale worker responses.

Integrated Chromium verification reproduced a visible edit that failed to save on the baseline. The repaired editor saves it to disk and retains it after reopening; switching rendered Markdown between two files replaces the previous content. The combined branch passed 303 focused tests and server/web typechecks.

Client verification evidence

b8-file-switching.webm

Model: GPT-6 Astra. Harness: Codex.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	+18 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+5 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+13 B (+0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	56.5 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.7 KiB	13.7 KiB	−5 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+4 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−9 B (−0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.3 KiB	57.3 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: a993745 · PR result: df4ae50 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.