test: cover guard race table directly

[edenbuilds]

Closes #8

Adds direct tests for the shared guard machine's race table:

• a round timer interrupting a pending phase step
• cancellation winning while the final step drains
• budget cutoff after exactly one overshooting step
• the configured consecutive-error retry cap

These tests exercise drivePhase and withErrorCap directly, complementing the existing whole-loop coverage without changing runtime behavior.

Verification:

• bun run check
• bun test (419 passed, 2 skipped live Claude smoke tests)