Reserve run IDs atomically

[Kubuxu]

Summary

• atomically reserve each generated run ID by creating its run directory
• retry when another process has already reserved the same ID
• propagate reservation failures during startup

Rationale

Random run ID collisions can occur in tests with an estimated probability of 1 in 1,600. Concurrent processes currently have no atomic claim step, so duplicate IDs can cause flaky tests and shared run state. Atomic directory creation removes that race.

This change was split out of #180 so it can be reviewed and merged separately.

Verification

• cargo test --lib (50 passed)

[Copilot]

Pull request overview

This PR improves the reliability of run ID generation by turning it into an atomic “claim” operation: a candidate run ID is only accepted if its corresponding run directory can be created, preventing concurrent processes from sharing the same run ID and run state.

Changes:

• Update generate_run_id to atomically reserve the run ID via directory creation, retrying on collisions.
• Add internal helpers for candidate generation and reservation, and adjust tests to validate reservation behavior.
• Propagate run ID reservation errors to the CLI entrypoint (main) via Result.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/run_id/mod.rs	Implements atomic run ID reservation using create_dir and updates tests to validate reserved IDs.
src/main.rs	Adapts to the new fallible generate_run_id() API by propagating errors with ?.

Suppressed comments (1)

src/run_id/mod.rs:146

• This test unconditionally imports std::os::unix::fs::symlink, which will fail to compile on Windows targets. Since create_latest_symlink has Windows support, the test should be gated to Unix (or implemented with platform-specific symlink creation).

    #[test]
    fn test_create_latest_symlink_handles_broken_symlinks() {
        // This test verifies that create_latest_symlink properly handles
        // removing broken symlinks and creating new ones
        use std::os::unix::fs::symlink;

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[Kubuxu]

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