phase1/stage1: bit-exact gate for the frame-scaled IDCT 4×4 #4
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Reference: marfrit/daedalus-decoder#4
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Stacks on PR #3. Adds test_idct_bitexact verifying GPU output is byte-for-byte identical to an inline C reference of the H.264 §8.5.12.1 butterfly across 4,800 random-coefficient blocks (300 MBs at 320×240). PASS first try on hertz. See commit message for what is and is not validated.
Adds test_idct_bitexact that exercises daedalus_decoder_flush_frame end-to-end with random coefficients and compares every output byte against an inline C reference of the H.264 §8.5.12.1 1D butterfly. Closes the validation gap from the previous PR ("dispatch succeeds" becomes "dispatch is bit-exact"). What's tested: - 320×240 coded frame (300 MBs), enough to cover multiple workgroups of the V3D shader (16 blocks/WG → ≥30 WGs) - Per-MB → flat-raster block layout consistent with flush_frame - Random coeffs in [-512, 511] (same range as daedalus-fourier cycle-6 M1 gate) - Inline C reference: H.264 §8.5.12.1 butterfly with column-major block layout, +32 rounding, >>6, add-to-predicted (=0), clip255 — mirrors daedalus-fourier tests/h264_idct4_ref.c Verified on hertz (Pi 5 / V3D 7.1 / daedalus-fourier 0.1.0): $ ctest --test-dir build --output-on-failure Start 1: smoke 1/2 Test #1: smoke ............................ Passed 0.16 sec Start 2: idct_bitexact 2/2 Test #2: idct_bitexact .................... Passed 0.03 sec 100% tests passed, 0 tests failed out of 2 Bit-exact PASS first try — daedalus-fourier's V3D IDCT 4x4 shader produces identical pixels to the C reference for all 4800 blocks in the test frame. Validates BOTH the shader correctness AND the frame-batched-dispatch correctness (this is the first time n_blocks > ~30 has been exercised at the recipe-dispatch layer; the substitution arc only ever called with n_blocks=1). What is NOT tested by this PR (deferred to follow-ons): - Non-zero predicted pixels — flush_frame zero-initialises scratch_y, so the IDCT-ADD reduces to clip255(IDCT). Real predicted comes from Stage 2a intra prediction. - Z-scan permutation between FFmpeg's per-MB coeffs layout and our per-MB → flat raster — the test uses its own coefficient generator that already matches our layout, so it doesn't exercise the permutation. The libavcodec-intercept patch is where the permutation lands and gets validated against real H.264 streams. - Chroma 4×4 IDCT. - IDCT 8×8 (High profile). Stacked on noether/phase1-stage1-idct (PR #3, the frame-scaled dispatch). Rebase on main after #3 lands; the diff is purely additive (one new test file + 5 lines of CMake).