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>

src/modules/discovery/Makefile

@@ -7,11 +7,13 @@ MODULE_BUILD = $(BUILD)/discovery
 
 all: $(MODULE_BUILD)/discovery.o
 
-$(MODULE_BUILD)/discovery.o: discovery.c $(ROOT)/include/discovery.h | $(MODULE_BUILD)
-	$(CC) $(CFLAGS) -c -o $@ $<
+$(MODULE_BUILD)/discovery.o: discovery.c | $(MODULE_BUILD)
+	$(CC) $(CFLAGS) $(DEPFLAGS) -c -o $@ $<
 
 $(MODULE_BUILD):
 	mkdir -p $@
 
 clean:
-	rm -f $(MODULE_BUILD)/discovery.o
+	rm -f $(MODULE_BUILD)/discovery.o $(MODULE_BUILD)/discovery.d
+
+-include $(MODULE_BUILD)/discovery.d