Add protocol module, video-node binary, query/web CLI tools

- Protocol module: framed binary encoding for control requests/responses
  (ENUM_DEVICES, ENUM_CONTROLS, GET/SET_CONTROL, STREAM_OPEN/CLOSE)
- video-node: scans /dev/media* and /dev/video*, serves V4L2 device
  topology and controls over TCP; uses UDP discovery for peer announce
- query_cli: auto-discovers a node, queries devices and controls
- protocol_cli: low-level protocol frame decoder for debugging
- dev/web: Express 5 ESM web inspector — live SSE discovery picker,
  REST bridge to video-node, controls UI with sliders/selects/checkboxes
- Makefile: sequential module builds before cli/node to fix make -j races
- common.mk: add DEPFLAGS (-MMD -MP) for automatic header dependencies
- All module Makefiles: split compile/link, generate .d dependency files
- discovery: replace 100ms poll loop with pthread_cond_timedwait;
  respond to all announcements (not just new peers) for instant re-discovery
- ENUM_DEVICES response: carry device_caps (V4L2_CAP_*) per video node
  so clients can distinguish capture nodes from metadata nodes

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

dev/cli/config_cli.c

@@ -62,7 +62,7 @@ int main(int argc, char **argv) {
 	}
 
 	if (!APP_IS_OK(err)) {
-		fprintf(stderr, "config_load: errno %d\n", err.detail.syscall.err_no);
+		app_error_print(&err);
 		return 1;
 	}