WP-D parity A/B. gpu_parity (ignored) decodes the conformance vector,
reads every frame back through the presenter's exact contract (wait,
layout round-trip, signal-back, release), crops at the copy so pitch
can never leak, and compares SHA-256s in display order against goldens
from ffmpeg software decode — cross-checked bit-identical between
ffmpeg 8.0.1 (linux) and 8.1.1 (macOS), so the reference is the spec,
not one build. PF_VKD_TEST_READBACK=1 is the one test-only hook (ORs
TRANSFER_SRC into pool usage; production pools stay zero-copy-tight).
Fleet verdict: 250/250 frames bit-identical to libavcodec on RADV
(Mesa 26.0.3, distinct), AMD proprietary Windows (25.10.30.02,
distinct) and NVIDIA Windows (610.88, coincide) — H.264 decode is
exactly specified, and the native path meets the spec on every driver
and both DPB arrangements.