canary: probe what an incremental admission rebuild recompiles (do not merge)

[teamleaderleo]

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

Why

In all 35 PR compile admissions since 11:00Z on 2026-09-24 that built tests, cmuxTests recompiled all ~1,091 of its files. That includes the runs where the app target cmux recompiled exactly one file, GeneratedAssetSymbols.swift, for example test-only PRs adopting a seed 1 to 3 commits back. On a 12vcpu runner that is about 2.5 min of a ~4.5 min compile, and more on 6vcpu. Main's own admissions (36006890597, 36002620252) show the same.

Two questions the logs cannot answer:

1. Why does GenerateAssetSymbols rerun on a seeded build? The seed replays file mtimes (e2e_warm_derived_data.replay) but not directory mtimes, and Assets.xcassets is a folder input.
2. Does re-emitting the cmux_DEV module (even with an unchanged interface) force a full cmuxTests rebuild?

What it does

After the normal admission compile, it rebuilds cmux-unit four times and records the recompiled files per target in the step summary:

• null rebuild
• mtime-only touch of one cmuxTests file
• mtime-only touch of the Assets.xcassets directory
• mtime-only touch of one app source file

🤖 Generated with Claude Code

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Summary by cubic

Adds a CI canary to probe why cmuxTests recompiles all ~1,091 files on incremental admission rebuilds even when the app target rebuilds a single file.

• After the normal admission compile, rebuilds cmux-unit four times — null, mtime-only touch of a test file, of Assets.xcassets, and of an app source file — and records recompiled file counts per target in the step summary.
• Times xcodebuild -resolvePackageDependencies with a warm manifest cache, no cache, and after a fresh cache rebuild to investigate the ~45 s package graph resolution.
• Probe logging runs with set +e so a failed build or empty grep does not abort the remaining probes.
• CMUX_CI_BUILD_SCHEMES now overrides the hardcoded scheme list in compile-app-host-test-product.sh.
• Do not merge; the PR will be closed once the admission job reports.

^{Written for commit d1565cd. Summary will update on new commits.}

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[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

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[github-actions]

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[teamleaderleo]

Measurement done; closing as planned. Its probes found the directory-mtime trigger (fixed in #14241), and the XCBuild trace from run 36022099083 led to #14262. Remaining cmuxTests rebuild work continues in #14269.