Make Swift helper compile cache safe under parallel builds

[thymikee]

What to build

Make Swift helper executable caching safe when multiple agent-device processes build the same cache key at the same time. This is most likely during parallel record stop or recording post-processing tests.

The fix should preserve the cache performance benefit while avoiding flaky rename or partial-artifact races.

Acceptance criteria

• Swift helper compilation writes to a unique temporary artifact before publishing the cache entry.
• Cache publication is atomic or protected so concurrent builders cannot leave a partial executable.
• If another process wins the cache race, the loser reuses the completed executable without failing.
• Tests simulate concurrent builds of the same cache key.
• Existing Swift script typecheck and recording tests continue to pass.

Blocked by

None - can start immediately