iter3 DEBUG: S_EXT_CTRLS EINVAL diagnostic in v4l2_ioctl_controls

When VIDIOC_S_EXT_CTRLS returns -EINVAL, log num_controls, error_idx,
and per-control id+size. Lets iter3+ debug "Unable to set control(s):
Invalid argument" failures by naming exactly which control set was
rejected — previously the request_log line in v4l2_set_controls just
printed strerror(errno) with no specificity.

Used in iter3 Phase 7 to confirm the frame-11 EINVAL is request-level
("error_idx == num_controls" sentinel = kernel rejected but couldn't
pinpoint a single field) rather than a single-control size mismatch.

To remove at iter4 DEBUG sweep alongside iter1 ENTER/CAPTURE-dump
instrumentation.

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

src/v4l2.c

@@ -433,6 +433,7 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 			       unsigned int num_controls)
 {
 	struct v4l2_ext_controls controls;
+	int rc;
 
 	memset(&controls, 0, sizeof(controls));
 
@@ -444,7 +445,16 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 		controls.request_fd = request_fd;
 	}
 
-	return ioctl(video_fd, ioc, &controls);
+	rc = ioctl(video_fd, ioc, &controls);
+	if (rc < 0 && errno == EINVAL && ioc == VIDIOC_S_EXT_CTRLS) {
+		request_log("S_EXT_CTRLS EINVAL: num_controls=%u error_idx=%u\n",
+			    num_controls, controls.error_idx);
+		for (unsigned int i = 0; i < num_controls; i++) {
+			request_log("  ctrl[%u]: id=0x%08x size=%u\n",
+				    i, control_array[i].id, control_array[i].size);
+		}
+	}
+	return rc;
 }
 
 int v4l2_get_controls(int video_fd, int request_fd,