Terminal batch can still be interrupted by gate-artifact writes and engine construction (#547 residuals)

Disclosed by the #548 round-4 lane and deliberately left out of that PR to keep its scope on the evidence-preservation guarantee. All three are 'the batch stops early', NOT 'the evidence is destroyed' — calibration_diagnostics.json is already on disk by the time any of them can fire, which is the #547 contract. Severity is therefore lower than the defects #548 fixes.

1. The three gate-artifact JSON writes are unguarded: input_coverage.json, input_mass_parity.json, qrf_tail_concentration.json are written with bare write_text inside the terminal-batch section (tools/build_us_fiscal_refresh_release.py). A disk-full/permission failure aborts main(), so the accumulated terminal_gate_failures never reach the terminal raise and later gate groups never run. #548 already secures each group's failure lines BEFORE its write, so the lines exist — they just are not reported. Fix shape: same treatment the telemetry proxy got, or a small guarded-write helper that records a line and continues.

2. PolicyEngineUSEngine() construction sits outside the degraded-mode guards in the same section: a construction failure aborts the batch.

3. Both are only reachable on a run that is already failing (or about to), so the practical exposure is a less complete failure report, not a wrong artifact.

Related: #547 (the original evidence-destruction defect), #548 (the fix), #543/#546 (checkpoint and cache identity).

Start in tools/build_us_fiscal_refresh_release.py at the terminal-batch section. Inspect the three gate-artifact writes and PolicyEngineUSEngine() construction alongside the existing degraded-mode handling and the related #548 changes. Done means failures in these operations are recorded while the batch continues to later gate groups and reaches the terminal failure report.