Remove stale circuit JSON before rebuilding

[ShiboSoftwareDev]

Summary

• remove an entrypoint's existing circuit.json before circuit generation starts
• apply the same behavior to sequential builds and concurrent worker builds
• preserve the source when a prebuilt Circuit JSON path is also the output path
• turn the reproduction into a regression test that requires the stale artifact to be absent after a fatal rebuild

Stack

• Reproduction: Reproduce stale circuit JSON after a failed rebuild #4394
• Fix: this PR

This branch is based directly on the reproduction branch so GitHub can show the stack.

Tests

• bun test tests/cli/build/build-failed-rebuild-stale-output.test.ts
• bun run format:check
• bun run build

[github-actions]

This PR has been automatically marked as stale because it has had no recent activity. It will be closed if no further activity occurs.

[github-actions]

This PR was closed because it has been inactive for 1 day since being marked as stale.