A file's mtime is unique, so cargo's freshness record is stable

[lannbot]

What

Each file's restored mtime takes a sub-second offset derived from its path, and scripts/restore-mtimes.py joins the build-directory cache key.

Why

#76 measured a steady gate of 3m12 with nothing compiled. On main it settled at 4m52 with eight units rebuilding every run — iroh, iroh-dns ×3, iroh-relay ×3, iroh-peer — from an exact-key cache hit. The branch's clean run was luck.

Cargo records a path dependency's freshness as the newest mtime in the package and the name of the file that carried it. A commit that touches several files gives them all one timestamp, so which file is newest is decided by directory order, which differs between clones. Running the gate under fingerprint logging (probe run 31740876568) names it exactly:

iroh-dns dirty: PrecalculatedComponentsChanged {
    old: "1784575660.000000000s (Cargo.toml)",
    new: "1784575660.000000000s (CHANGELOG.md)" }
iroh-relay dirty: PrecalculatedComponentsChanged { ... (Cargo.toml) → (CHANGELOG.md) }
iroh dirty:      UnitDependencyInfoChanged { ... }   ← the cascade
iroh-peer dirty: UnitDependencyInfoChanged { ... }

Same instant, different file. A path-derived nanosecond offset makes one file strictly newest, and every clone agrees which. Verified locally: every package under .deps/iroh now has exactly one newest file (iroh-dns previously tied Cargo.toml with CHANGELOG.md, precisely as cargo reported).

Measured

Seeded, then two consecutive runs on this branch — each re-clones .deps, so each walks a differently ordered directory:

run	compiled	gate step
31741582706 (seeds the key)	9	—
31742095487	0	2 m 42 s
31742497596	0	2 m 44 s

Against 4m52 with eight rebuilds on main today. just check passes; this PR's own run is the gate.

Notes

• The key gains scripts/restore-mtimes.py: artifacts are kept on the strength of the mtimes they were judged fresh against, so changing how those are assigned must invalidate them. Removing the temporary branch allowlist used for the measurement changes ci.yml, which is in the key too — so the first run on main restores the newest entry under the prefix (not cold), rebuilds those units once, and saves. The run after that is the steady state above.
• Verification allowlist stripped; the branch is one commit.