feat: node-to-node MJPEG streaming CLIs and shared V4L2 format header

Add stream_send_cli (V4L2 capture → TCP → VIDEO_FRAME) and
stream_recv_cli (TCP → threaded frame slot → GLFW display) to
exercise end-to-end streaming between two nodes on the same machine
or across the network.

Add include/stream_stats.h (header-only rolling-window fps/Mbps tracker)
and include/v4l2_fmt.h (header-only V4L2 format enumeration shared between
v4l2_view_cli and stream_send_cli). Refactor v4l2_view_cli to use the
shared header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

include/stream_stats.h

@@ -0,0 +1,76 @@
+#pragma once
+
+/*
+ * Lightweight per-stream statistics tracker.
+ * Header-only; include wherever stream send/receive happens.
+ *
+ * Usage:
+ *   Stream_Stats s;
+ *   stream_stats_init(&s, stream_id);
+ *
+ *   // on each frame:
+ *   stream_stats_record_frame(&s, byte_count);
+ *
+ *   // periodically (e.g. after every frame):
+ *   if (stream_stats_tick(&s)) {
+ *       printf("%.1f fps  %.2f Mbps\n", s.fps, s.mbps);
+ *   }
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include <time.h>
+
+#define STREAM_STATS_INTERVAL 0.5   /* recompute rates every 0.5 s */
+
+typedef struct {
+	uint16_t        stream_id;
+
+	/* Lifetime counters — never reset. */
+	uint64_t        total_frames;
+	uint64_t        total_bytes;
+
+	/* Rolling window — reset each time rates are computed. */
+	uint64_t        window_frames;
+	uint64_t        window_bytes;
+	struct timespec window_start;
+
+	/* Last computed rates. */
+	float           fps;
+	float           mbps;
+} Stream_Stats;
+
+static inline void stream_stats_init(Stream_Stats *s, uint16_t stream_id)
+{
+	memset(s, 0, sizeof(*s));
+	s->stream_id = stream_id;
+	clock_gettime(CLOCK_MONOTONIC, &s->window_start);
+}
+
+/* Call once per received/sent frame. */
+static inline void stream_stats_record_frame(Stream_Stats *s, uint32_t nbytes)
+{
+	s->total_frames++;
+	s->total_bytes  += nbytes;
+	s->window_frames++;
+	s->window_bytes += nbytes;
+}
+
+/*
+ * Recompute fps and mbps if enough time has elapsed.
+ * Returns 1 when rates were updated, 0 otherwise.
+ */
+static inline int stream_stats_tick(Stream_Stats *s)
+{
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	double elapsed = (double)(now.tv_sec  - s->window_start.tv_sec) +
+	                 (double)(now.tv_nsec - s->window_start.tv_nsec) * 1e-9;
+	if (elapsed < STREAM_STATS_INTERVAL) { return 0; }
+	s->fps  = (float)((double)s->window_frames / elapsed);
+	s->mbps = (float)((double)s->window_bytes * 8.0 / elapsed / 1e6);
+	s->window_frames = 0;
+	s->window_bytes  = 0;
+	s->window_start  = now;
+	return 1;
+}