wip: predicted samples plumbing

include/daedalus_decoder.h

@@ -89,6 +89,26 @@ struct daedalus_decoder_mb_input {
      * column-major within each 4x4 or 8x8 block (matches FFmpeg
      * convention).  Caller-owned; copied during append. */
     const int16_t *coeffs;       /* points at exactly 384 int16_t */
+
+    /* Reconstructed predicted samples for this MB, planar order:
+     *   [  0 .. 256) — 16×16 luma, ROW-MAJOR raster (row 0 cols 0..15,
+     *                  row 1 cols 0..15, ..., row 15 cols 0..15)
+     *   [256 .. 320) — 8×8 Cb, ROW-MAJOR raster
+     *   [320 .. 384) — 8×8 Cr, ROW-MAJOR raster
+     *
+     * The caller (libavcodec's CPU intra-prediction kernels for Phase 1
+     * I-frames; MC fallback for Phase 2 P-frames before GPU MC lands)
+     * populates this from neighbour samples per H.264 §8.3 / §8.4.
+     * `flush_frame()`'s reconstruction step is `clip255(predicted +
+     * idct(coeffs))` — the IDCT shader reads dst, adds the inverse
+     * transform, writes clipped — so a non-zero `predicted` here makes
+     * the output pixel a valid H.264 reconstruction; zero means
+     * residual-only (used by IDCT-isolation tests).
+     *
+     * NULL is legal and means "all-zero predicted samples" for this MB
+     * (the per-frame predicted buffer is zeroed at flush time so a NULL
+     * is indistinguishable from explicit zeros). */
+    const uint8_t *predicted;    /* NULL or exactly 384 uint8_t */
 };
 
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