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.c

@@ -254,6 +254,7 @@ VAStatus RequestCreateContext(VADriverContextP context, VAConfigID config_id,
 		goto error;
 	}
 	memset(&context_object->dpb, 0, sizeof(context_object->dpb));
+	context_object->timestamp_counter = 0;	/* iter9 α-7 */
 
 	/*
 	 * Initialize the OUTPUT (bitstream-input) buffer pool. Sized by