feat(pair): refuse a rewind while the executor is working

[rynfar]

What

Web and mobile refuse to rewind a lead while its pair executor holds a turn, and say why: "Stop the executor before rewinding. It works in this thread's worktree and would write over the restored files."

Why

A lead and its executor share one worktree. The server already interrupts a running executor when a rewind is requested, but it says so itself: the executor can still write for a moment before the interrupt lands, onto files the rewind is restoring. Changing rewind admission in the decider was ruled out to keep pair mode out of the core, so the clients ask first and the reactor stays as the backstop.

How

• pairRewindBlockedReason in client-runtime/state/pair.ts (pure): blocked while the executor is starting, running, or paused on an approval or a question, since it resumes and writes once answered. Not blocked when the pair is off, the executor is idle or finished, or the thread is another lead's or another environment's.
• Web: ChatView's rewind handler checks it right after the existing "interrupt the current turn first" check and reports it the same way. That is the only change to ChatView.
• Mobile: ThreadRouteScreen shows it as a "Rollback unavailable" alert before the confirmation.

Verification

• vp test run packages/client-runtime/src/state: all passing, with two new tests written first (one seen failing; the "allows it" cases pass against the stub by construction).
• Typecheck @t3tools/client-runtime, @t3tools/web, @t3tools/mobile: 0 errors. Format clean; no new lint warnings in the two touched screens.
• Not run in a browser or on a simulator.

Part of #622.

Model: Claude Fable 5.1, in Pylon via the Claude Code harness.

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[vercel]

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Project	Deployment	Actions	Updated
pylon-marketing	Ready	Preview	Sep 18, 2026 10:15pm UTC

[vercel]

Deployment failed for project pylon-marketing with the following error:

Resource is limited - try again in 24 hours (more than 100, code: "api-deployments-free-per-day").

Learn More: https://vercel.com/pylon-code?upgradeToPro=build-rate-limit

[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	14.0 KiB	14.0 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	+3 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−1 B (−0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	14.0 KiB	13.9 KiB	−18 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	−7 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−11 B (−0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: e397226 · PR result: 29445f3 · 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: 115.6 KiB
• Claude decoded thread snapshot: 116.4 KiB

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

[rynfar]

Merging with the Vercel preview check red: it failed with build-rate-limit (the account's build quota, tracked in #655), not because of this change. Every code check is green.