canary: measure checksum-only rebuilds at cmux scale (do not merge)

[teamleaderleo]

Measurement canary for ci/seed-module-reemit. Do not merge; I'll close it once the admission job reports.

It builds with the fix (Swift Build FileSystemMode=checksum-only for the admission builds), then rebuilds three times in the same job:

• null rebuild: what content hashing costs when nothing changed
• every checkout file, directory and generated module map touched with the same bytes, which is what a fresh runner adopting a seed looks like; it also tries to touch Xcode SDK headers
• one app source edit

The fingerprint changes with the mode, so this admission misses the seed and compiles cold under checksum-only.

🤖 Generated with Claude Code

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Summary by cubic

Adds a measurement canary for the checksum-only seed fix. Do not merge; it will be closed once the admission job reports.

The admission build now runs Swift Build with FileSystemMode=checksum-only so llbuild compares inputs by content. Without it, an adopted seed at distance 0 still reran 94 SwiftDriver and 64 SwiftEmitModule tasks because 1,616 Xcode SDK and toolchain files carry different mtimes across Blacksmith images, and generated package module maps get rewritten with identical bytes on the first build after adoption. The mode is set only in the environment of these xcodebuild invocations and joins the seed fingerprint, since a database written in one mode reruns everything in the other.

• Probes three rebuilds in the admission job: a null rebuild, every checkout file and module map touched with the same bytes plus SDK headers, and one app source edit.
• Adds tests asserting every scheme builds in checksum-only mode and the fingerprint names the mode.

^{Written for commit 18b36db. Summary will update on new commits.}

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[teamleaderleo]

Canary done (run 36029379921): numbers are in #14262. Closing; do not merge.