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>

Makefile

@@ -1,9 +1,25 @@
-.PHONY: all clean cli
+ROOT := $(abspath .)
+include $(ROOT)/common.mk
 
-all: cli
+.PHONY: all clean modules cli node
 
-cli:
+all: modules cli node
+
+modules:
+	$(MAKE) -C src/modules/common
+	$(MAKE) -C src/modules/media_ctrl
+	$(MAKE) -C src/modules/v4l2_ctrl
+	$(MAKE) -C src/modules/serial
+	$(MAKE) -C src/modules/transport
+	$(MAKE) -C src/modules/discovery
+	$(MAKE) -C src/modules/config
+	$(MAKE) -C src/modules/protocol
+
+cli: modules
 	$(MAKE) -C dev/cli
 
+node: modules
+	$(MAKE) -C src/node
+
 clean:
 	rm -rf build/