iter4 Phase 3: VP9 baseline + 4-codec regression on 7.0 substrate

Captured on linux-fresnel-fourier 7.0-1 (post 6.19 decommission).

VP9 baseline (kernel-direct via ffmpeg-v4l2request on rkvdec):
- 5-frame SW reference PNG SHA256 anchors (criterion-4)
- VIDIOC_S_EXT_CTRLS strace with full payload at -s 16384
- Empirical struct sizes 168 B (FRAME) / 2040 B (COMPRESSED_HDR)
  supersede Phase 2 estimates of 144 / 1947
- Probe pattern: count=1 (FRAME-only) then count=2 (FRAME + COMPRESSED_HDR)

Phase 2 doc fix: control IDs corrected 0xa40b2c/d -> 0xa40a2c/d.

4-codec regression (H.264, MPEG-2, HEVC, VP8): all fall back to SW on
default config because /dev/video0 is now rockchip-rga (RGB color
converter), not a codec device. Fork hardcodes /dev/video0 in
request.c:149. Env override LIBVA_V4L2_REQUEST_VIDEO_PATH /
_MEDIA_PATH restores per-driver profile enumeration; mitigation A/B/C
queued for user decision.

New contract clauses surfaced:
- Clause 11: uncompressed-header partial parse for lf_delta /
  base_q_idx (VAAPI doesn't expose these; keyframe ref_deltas non-zero
  for BBB so leave-at-zero is wrong)
- Clause 12: compile-time sizeof asserts on the two control structs
  so future UAPI shifts fail loudly

iter4_phase3.tgz: full Phase 3 artifact bundle (strace + PNG refs).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

phase2_iter4_situation.md

@@ -149,7 +149,7 @@ Per memory `feedback_runtime_enumerates_allowlists.md`: plan for fix-forward Com
 
 ```c
 #define V4L2_CID_STATELESS_VP9_FRAME        (V4L2_CID_CODEC_STATELESS_BASE + 300)
-                                            /* = 0xa40b2c */
+                                            /* = 0xa40a2c */
 
 struct v4l2_ctrl_vp9_frame {
     struct v4l2_vp9_loop_filter lf;        /* 16 bytes; ref_deltas[4] + mode_deltas[2]
@@ -187,7 +187,7 @@ Total size: ~144 bytes (vs iter3 VP8's 1232 bytes — much smaller because VP9_F
 
 ```c
 #define V4L2_CID_STATELESS_VP9_COMPRESSED_HDR  (V4L2_CID_CODEC_STATELESS_BASE + 301)
-                                              /* = 0xa40b2d */
+                                              /* = 0xa40a2d */
 
 struct v4l2_ctrl_vp9_compressed_hdr {
     __u8 tx_mode;                          /* V4L2_VP9_TX_MODE_* (0..4) */