iter9 Phase 6 α-7: monotonic per-context timestamp counter

Replace gettimeofday in RequestEndPicture with object_context-scoped
counter producing small us values (1, 2, 3, ...) so OUTPUT QBUF
timestamp and DPB.reference_ts match ffmpeg-v4l2request's pattern.

Phase 5 IMP-1: counter scoped to object_context (not driver_data) to
avoid multi-context collisions.

Empirical confirmation only — reviewer's CRIT-1 predicts this is
inert (VP9/MPEG-2 use same path and PASS). If α-7 produces the same
broken hash, the libva wire-byte search space is exhausted and iter10
must pivot to slice-data inspection or kernel investigation.

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

src/context.h

@@ -55,6 +55,15 @@ struct object_context {
 	struct h264_dpb dpb;
 	bool h264_start_code;
 
+	/*
+	 * iter9 α-7: monotonic per-context timestamp counter (us). Replaces
+	 * gettimeofday in EndPicture so DPB.reference_ts / OUTPUT QBUF ts
+	 * are small values matching ffmpeg-v4l2request's pattern. Placed
+	 * here (object_context) not driver_data per Phase 5 IMP-1 to avoid
+	 * cross-context collisions.
+	 */
+	uint64_t timestamp_counter;
+
 	/* fresnel-fourier iter4: VP9 loop-filter delta state, persisted across
 	 * frames per kernel UAPI <linux/v4l2-controls.h>:2578 ("If this syntax
 	 * element is not present in the bitstream, users should pass its last