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>

common.mk

@@ -2,6 +2,11 @@
 # Each including Makefile must set ROOT before including this file:
 #   ROOT := $(abspath <relative path to repo root>)
 
-CC     = gcc
-CFLAGS = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
-BUILD  = $(ROOT)/build
+CC      = gcc
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
+BUILD   = $(ROOT)/build
+
+# Automatic dependency generation.
+# -MMD  emit a .d file listing header prerequisites alongside each .o
+# -MP   add phony targets for headers so removed headers don't break make
+DEPFLAGS = -MMD -MP