[Bench] Fixes for pure Fizz bench

[gaearon]

Summary

Harness fixes, all surfaced while benching a Fizz-only React PR (react/react#37022):

• Profile pass now alternates arm order by VM index. The timed runs are ABBA, but the CPU profile pass ran base-then-cand on every VM. The second arm runs warmer (page cache, governor), which deflated every function in the cand profile by a uniform few percent — untouched Flight frames "improved" ~7%. With balanced order the same untouched frames diff to ~1.5% across 14 paired VMs while real movers keep their magnitude. Each VM records its capture order in the profiles tarball (prof-order.txt) so analyses can check order drift directly.
• The fingerprint now covers every server-side React layer. FP_FILES hashed only the vendored Flight server files, which a Fizz-only React change leaves byte-identical — both arms of a valid A/B reported the same fingerprint. Added the vendored Fizz bundle and the shared react-server runtime (hooks/cache) in both suites. The VM boot loop also warns loudly when arms fingerprint identically, instead of leaving it for the analysis footer.
• --prepare writes a terminal status phase. It previously exited leaving phase: "measuring", so bench-status prescribed collecting measurement data that never existed. bench-status now reports prepare-only runs as complete rather than "DEAD — relaunch".

Verification

• Full 16-VM e2e run with the ordering + Fizz-fingerprint changes (run-pr37022-e2e-prof2): arms fingerprint distinctly (18ea275e vs f9805be5), profile order alternates per VM, untouched hot frames diff to ~zero in the cross-VM profile aggregate.
• bench-status.mjs against a real --prepare run dir reports complete (--prepare: caches only).
• --dry-run smoke of the modified launcher; node --check on all three scripts.

[github-actions]

Tests Passed

Commit: cf8a1ce

[github-actions]

Stats skipped

Commit: cf8a1ce
View workflow run