[raphaelnz]

Why

Keep upstream synchronization from accepting a tree it could not compare. The reproducibility check currently treats empty diff output as success, including read errors.

Closes #47.

What changed

• Check each tree comparison's exit status and retain its diagnostics.
• Fail reproducibility if any scope differs or cannot be compared, even if a later scope succeeds.
• Add regression cases and document the failure behavior.

Scope

• Covers the reproducibility check and its regression tests.
• Does not change release versions, sync recovery, or upstream target selection.

Blast Radius

The port gate now rejects comparisons that previously returned a false PASS. Successful comparisons keep the existing behavior.

Verification

• Real GNU diff ran as uid 65534 against a same-size modified file with mode 000. The reproducibility check reported FAIL and the gate exited 1 with the permission error.
• The new mutation tests fail against the unfixed gate. The fixed full port gate passes, including all 35 mutation cases and the worktree audit.
• Shell syntax checks pass for both changed scripts.