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>

include/error.h

@@ -14,8 +14,8 @@ struct Syscall_Error_Detail {
 };
 
 struct Invalid_Error_Detail {
-	/* fields added as concrete cases arise */
-	int placeholder;
+	int         config_line;   /* source line number, or 0 if not applicable */
+	const char *message;       /* static string describing what was wrong     */
 };
 
 struct App_Error {
@@ -51,6 +51,14 @@ void app_error_print(struct App_Error *e);
 		.line = __LINE__, \
 	})
 
+#define APP_INVALID_ERROR_MSG(cfg_line, msg) \
+	((struct App_Error){ \
+		.code   = ERR_INVALID, \
+		.file   = __FILE__, \
+		.line   = __LINE__, \
+		.detail = { .invalid = { .config_line = (cfg_line), .message = (msg) } }, \
+	})
+
 #define APP_NOT_FOUND_ERROR() \
 	((struct App_Error){ \
 		.code = ERR_NOT_FOUND, \