fix(cmux-tui): fence repeated ABA echo keys

[lawrencecchen]

Stacked follow-up for PR #11002.

The durable echo key previously hashed only the current semantic state and its predecessor. A repeated Working → Blocked → Working → Blocked cycle reused the earlier Blocked key and could replay or reject the later transition.

This adds a durable monotonic resource revision to the transition key. Immediate identical reports still coalesce before key generation.

Commits:

• 881a3ac adds the failing repeated-ABA behavior test.
• e378a13 adds the fix.

Validation: git diff --check; hosted cmux-tui verification is required because local Cargo/rustc is prohibited.

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Summary by cubic

Fixes repeated ABA echo key reuse in cmux-tui by including a durable monotonic resource revision in the transition key, so repeated Working → Blocked → Working cycles no longer reuse an earlier key and can replay or reject the later transition.

^{Written for commit 3a62c0e. Summary will update on new commits.}

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Project	Deployment	Actions	Updated
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[lawrencecchen]

Closing as superseded by current main (6d67bc5). The PR relies on the old agent-report-echo-v4 transition-key path in mux.rs. Current main replaced that path with durable resource mutation revisions and hook-fence checks, so the ABA hunk has no target context after refresh. Current main also carries lifecycle-fence coverage for delayed old session starts and late events. Reapplying these three commits onto current main cannot produce a safe, narrow patch.