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/node/Makefile

@@ -0,0 +1,42 @@
+ROOT := $(abspath ../..)
+include $(ROOT)/common.mk
+
+NODE_BUILD    = $(BUILD)/node
+MAIN_OBJ      = $(NODE_BUILD)/main.o
+COMMON_OBJ    = $(BUILD)/common/error.o
+MEDIA_OBJ     = $(BUILD)/media_ctrl/media_ctrl.o
+V4L2_OBJ      = $(BUILD)/v4l2_ctrl/v4l2_ctrl.o
+SERIAL_OBJ    = $(BUILD)/serial/serial.o
+TRANSPORT_OBJ = $(BUILD)/transport/transport.o
+DISCOVERY_OBJ = $(BUILD)/discovery/discovery.o
+CONFIG_OBJ    = $(BUILD)/config/config.o
+PROTOCOL_OBJ  = $(BUILD)/protocol/protocol.o
+
+.PHONY: all clean
+
+all: $(NODE_BUILD)/video-node
+
+$(NODE_BUILD)/video-node: $(MAIN_OBJ) \
+	$(COMMON_OBJ) $(MEDIA_OBJ) $(V4L2_OBJ) $(SERIAL_OBJ) \
+	$(TRANSPORT_OBJ) $(DISCOVERY_OBJ) $(CONFIG_OBJ) $(PROTOCOL_OBJ)
+	$(CC) $(CFLAGS) -o $@ $^ -lpthread
+
+$(MAIN_OBJ): main.c | $(NODE_BUILD)
+	$(CC) $(CFLAGS) $(DEPFLAGS) -c -o $@ $<
+
+$(COMMON_OBJ):    ; $(MAKE) -C $(ROOT)/src/modules/common
+$(MEDIA_OBJ):     ; $(MAKE) -C $(ROOT)/src/modules/media_ctrl
+$(V4L2_OBJ):      ; $(MAKE) -C $(ROOT)/src/modules/v4l2_ctrl
+$(SERIAL_OBJ):    ; $(MAKE) -C $(ROOT)/src/modules/serial
+$(TRANSPORT_OBJ): ; $(MAKE) -C $(ROOT)/src/modules/transport
+$(DISCOVERY_OBJ): ; $(MAKE) -C $(ROOT)/src/modules/discovery
+$(CONFIG_OBJ):    ; $(MAKE) -C $(ROOT)/src/modules/config
+$(PROTOCOL_OBJ):  ; $(MAKE) -C $(ROOT)/src/modules/protocol
+
+$(NODE_BUILD):
+	mkdir -p $@
+
+clean:
+	rm -f $(NODE_BUILD)/video-node $(MAIN_OBJ) $(NODE_BUILD)/main.d
+
+-include $(NODE_BUILD)/main.d