Arm builds from commits, cached, with Harness versions fixed #1002

Parent

#977

What to build

• Building: for each Arm, thirdshift builds itself from the Arm's commit, with cargo build --release --locked in a worktree.
• Caching: builds are cached by commit under ~/.thirdshift/evals/builds/, so the same commit is built once. The Experiment records each Arm's commit, since thirdshift version can't tell a build from main apart from the last release.
• Harness versions: when an Experiment starts, it resolves each Harness's binary to its real path and records its version. Every Trial then runs on that binary, so a Harness that updates itself partway through can't reach one Arm and not the other.
• Tests seed the cache, and build nothing.

Acceptance criteria

• A commit already in the cache isn't built again.
• A failed build stops the Experiment before its first Trial, with the build's error.
• Tests seed the cache with the test binary for both Arms' commits.
• Each Harness's resolved path and version are recorded. Trials keep using that path even when the Harness's link changes mid-Experiment, tested with a fake Harness whose link is swapped.