perf(web): defer draft saves and preserve reloaded annotations

[rynfar]

Typing in one composer serialized every saved draft and attachment before the storage debounce ran. Draft reload also dropped preview annotations, despite writing them to storage.

Adopt upstream #9695 (dab5f6e6e02e78675655e69503aa89654e5b8050) to defer the draft walk and serialization until the save flushes. Preserve Pylon’s version-10 draft schema, explicit provider choices, binding-conflict barrier and synchronous flush before navigation. Restore valid preview annotations during normalization and retain annotation-only drafts.

Pylon’s two existing conflict-recovery tests accidentally read current memory. They now flush storage, clear memory and rehydrate. That stronger check exposed the annotation loss and fails without the normalization fix. A new case covers annotation-only drafts and malformed saved annotations.

Validation: all 107 composer tests, web typecheck, changed-file lint and formatting passed. Tests cover deferred serialization, canceled saves, final flushes, legacy migration, attachments, citations, provider conflicts and complete draft transfers. No stored-version change or mobile persistence change. Rendering and debounce duration are unchanged; no browser performance benchmark is claimed.

Model: GPT-6 Astra. Harness: Codex.

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[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	13.8 KiB	+177 B (+1.3%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−8 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.7 KiB	+185 B (+2.8%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	57.4 KiB	+910 B (+1.6%)	66.4 KiB	✅
Codex	Live turn messages	8	10	+2 (+25.0%)	21	✅
Claude	Total thread wire	13.7 KiB	13.8 KiB	+151 B (+1.1%)	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	6.7 KiB	+155 B (+2.3%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.4 KiB	58.2 KiB	+884 B (+1.5%)	66.4 KiB	✅
Claude	Live turn messages	9	10	+1 (+11.1%)	21	✅

Baseline: 3e87857 · PR result: 7082702 · 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.