canary: measure what one app edit costs cmuxTests, cache on and off (do not merge)

[teamleaderleo]

Measurement canary. Do not merge; I'll close it once the admission job reports.

Why

A PR that changes the app module cmux (cmux_DEV) usually recompiles all 1,091 cmuxTests files: 129 s median on 12vcpu, 199 s on 6vcpu. #14235 showed that an mtime-only touch of an app file is cheap (50 s) and that the asset-timestamp case (fixed by #14241) is a real full rebuild. It did not measure real content edits.

What it does

After the normal admission compile, it rebuilds cmux-unit in three series, each a baseline, a null rebuild, then cumulative edits:

1. app function body only
2. new private func in that file
3. new internal method on GPUSpinner (one test file names it)
4. signature change of that method
5. new internal method on Workspace (239 test files name it)
6. body-only edit of one cmuxTests file

Series: cached (today's CI settings), uncached (COMPILATION_CACHE_ENABLE_CACHING=NO, explicit modules on, -driver-show-incremental), uncached-implicit (also SWIFT_ENABLE_EXPLICIT_MODULES=NO). The step summary reports per-target span, SwiftCompile counts, timing summary rows and incremental remarks; raw logs are uploaded.

🤖 Generated with Claude Code

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Summary by cubic

Measurement canary only—do not merge. Adds a post-compile probe that rebuilds cmux-unit after cumulative app and test edits to measure cmuxTests incremental costs with its compilation cache enabled, disabled, and disabled with explicit modules off.

• Covers body, private member, narrow and wide internal member, signature, and test-body edits, plus builds after cmux-numeric-locale.
• Runs targeted cache-off probes for cmuxTests while leaving other targets cached, records incremental remarks for cached rebuilds, and adds a cached-short control series.
• Pins the macOS job to 12-vcpu macOS 26 and raises the unit-batch timeout to 240 minutes.
• The probe is non-blocking, raw logs are uploaded, and probe logs are kept on timeout.

^{Written for commit 30293f8. Summary will update on new commits.}

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[teamleaderleo]

Measurements done. Results and the fix are in #14349.