fix(client): retain failed thread-load diagnostics

[rynfar]

A thread load could terminate during initialization or protocol handling without retaining a useful diagnostic. Later connection notifications or buffered events could erase the failure even though no new load had begun.

Observe the complete subscription attempt and retain a generic diagnostic until an actual retry starts. Preserve transport/domain recovery, fatal-defect behavior, deletion precedence, and Pylon's rollback and pagination handling. This manually adapts upstream #10216, bd16b86d50c1df49afeb7c0a7568a4908ade4048, without importing its unrelated subscription architecture.

Validation: 49 focused RPC, thread failure, synchronization, pagination, and atom tests passed. Shared client-runtime and mobile typechecks, targeted lint, format, and diff checks passed. The shared state applies across providers and connection modes. Existing uncached mobile error UI can display the diagnostic; this does not add web/desktop error presentation or restart providers.

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	13.6 KiB	+24 B (+0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−2 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+26 B (+0.4%)	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	13.8 KiB	13.7 KiB	−98 B (−0.7%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−1 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.6 KiB	−97 B (−1.4%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	57.3 KiB	−840 B (−1.4%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: a2fb9c9 · PR result: 7e319f7 · 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.