GCFT-0438: The Sealed Loop — Blinded One-Loop BF Pole Execution and the Broken-Phase Threshold Gate

GCFT-0438 executes the first blinded one-loop quantum step under the frozen GCFT-0437 endpoint-blinding contract. It confirms that the pure Xi phase channel remains zero at retained order, derives the local BF tadpole pole symbolically, and records its cancelling MSbar counterterm while keeping the precision endpoint sealed. The result is a partial execution, not a promotion: full dimension-six BRST operator mixing, finite broken-phase threshold matching, and scale-stability checks remain open gates before any endpoint comparison can be reported.