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>

README.md

@@ -18,6 +18,8 @@ Designed to run on resource-constrained hardware (Raspberry Pi capturing raw MJP
 - `test_image_cli` — generate test patterns and write PPM output for visual inspection
 - `xorg_cli` — display a test pattern in the viewer window; exercises scale/anchor modes and text overlays
 - `v4l2_view_cli` — live camera viewer; auto-selects highest-FPS format, displays FPS overlay
+- `stream_send_cli` — capture MJPEG from V4L2, connect to a receiver over TCP, stream VIDEO_FRAME messages with per-stream stats
+- `stream_recv_cli` — listen for incoming TCP stream, display received MJPEG frames with fps/Mbps overlay
 
 ## Structure