iter4 DEBUG: per-control VIDIOC_TRY_EXT_CTRLS isolation

Iterates each control individually through VIDIOC_TRY_EXT_CTRLS on
S_EXT_CTRLS EINVAL. Used in iter4 Phase 4 to diagnose the carryover
frame-11 EINVAL: discovered all four H.264 controls fail individually
on the same request_fd → diagnosis pivot from "bad control content"
to "bad request_fd state," which led to the fresh-request_fd-per-frame
fix in 385dee1.

Stays in for the iter5 DEBUG sweep alongside iter1 ENTER traces +
iter3 Y2 + iter4 Y2v3 + iter4 DPB census.

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

src/v4l2.c

@@ -473,15 +473,29 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 		if (try_rc < 0) {
 			request_log("  TRY_EXT_CTRLS retry: errno=%d (%s) error_idx=%u\n",
 				    errno, strerror(errno), try_controls.error_idx);
-			if (try_controls.error_idx < num_controls) {
-				request_log("  --> failing control is ctrl[%u]: id=0x%08x size=%u\n",
-					    try_controls.error_idx,
-					    control_array[try_controls.error_idx].id,
-					    control_array[try_controls.error_idx].size);
-			}
 		} else {
 			request_log("  TRY_EXT_CTRLS retry: passed (S vs TRY semantics differ — likely cluster commit)\n");
 		}
+		/* Per-control isolation: TRY each control alone to learn which
+		 * one fails on its own. Exposes per-control compound validators
+		 * (std_validate_compound) below the cluster commit obfuscation.
+		 */
+		for (unsigned int i = 0; i < num_controls; i++) {
+			struct v4l2_ext_controls one_ctrl;
+			memset(&one_ctrl, 0, sizeof(one_ctrl));
+			one_ctrl.controls = &control_array[i];
+			one_ctrl.count = 1;
+			if (request_fd >= 0) {
+				one_ctrl.which = V4L2_CTRL_WHICH_REQUEST_VAL;
+				one_ctrl.request_fd = request_fd;
+			}
+			int one_rc = ioctl(video_fd, VIDIOC_TRY_EXT_CTRLS, &one_ctrl);
+			request_log("  TRY[%u] id=0x%08x: %s (errno=%d error_idx=%u)\n",
+				    i, control_array[i].id,
+				    one_rc < 0 ? "FAIL" : "pass",
+				    one_rc < 0 ? errno : 0,
+				    one_ctrl.error_idx);
+		}
 		errno = saved_errno;
 	}
 	return rc;