ci(cmux-tui): cache cargo builds in the Rust jobs

[teamleaderleo]

cmux-tui.yml had no cargo cache, so every dispatch compiled the whole workspace from scratch. This adds Swatinem/rust-cache (pinned to v2.9.2) after "Set up pinned Rust" in the lint, valgrind and test jobs, caching the registry and cmux-tui/target. Caches stay scoped to the dispatched branch, so no branch writes into main's scope.

Measured (focused mode, unix_process_scope, Blacksmith Linux)

	test (linux)	cargo test step	lint (linux)
No cache (3 recent runs)	839 to 959 s (mean 907)	614 s	233 to 238 s
This branch, cold (run 36132320918)	831 s		244 s
This branch, warm (run 36134437399)	691 s	426 s	175 s

Agents re-dispatch this workflow on the same branch while iterating (verify-cmux-tui-hosted.sh), so from the second run on each iteration saves about 3.5 min per test job and 1 min on lint. The cache restore is fast enough that it doesn't show up among the slowest steps.

🤖 Generated with Claude Code

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Summary by cubic

Adds cargo caching to the cmux-tui CI jobs so the workspace no longer recompiles from scratch on every dispatch.

• Swatinem/rust-cache (v2.9.2) runs after the pinned Rust setup in the lint, valgrind, and test jobs, caching the registry and cmux-tui/target.
• Caches scope to the dispatched branch, so reruns on the same branch build incrementally and branch caches never reach main.
• Warm runs measured at 691 s for test (down from ~921 s) and 175 s for lint, saving about 3.5 min per test job and 1 min on lint from the second run on.

^{Written for commit ba772f3. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 561d317 and ba772f3.

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• .github/workflows/cmux-tui.yml

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[github-actions]

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[github-actions]

Merge receipt for ba772f3884, merged 2026-09-25 12:39:12 UTC

• Not verified at merge: ci-status (not reported), guards (17) (in progress), seven-language live conformance (in progress)
• Verified: Web complexity, web-validation, Fast static checks, GhosttyKit release check, protocol contract, Testbox broker trust boundary
• Skipped by policy: ${{ matrix.language }} consumer, ${{ matrix.language }} package, browser, Claude wrapper regressions, macos, remote-daemon, Rust SDK MSRV (1.88), suite-coverage, web, web-build, web-database-tests, web-tests
• Full suite: runs on main after merge.

Labeled merged-unverified: if main breaks near this merge, look here first.