Add discovery module: UDP multicast announcements and peer tracking

Sends 6-byte framed announcements to 224.0.0.251:5353 on startup and
every interval_ms (default 5s). Receive thread maintains a peer table
(max 64 entries); fires on_peer_found for new peers, on_peer_lost when
a peer misses timeout_intervals (default 3) consecutive intervals.

Own announcements are filtered by name+site_id. SO_REUSEADDR+REUSEPORT
allows multiple processes on the same host for testing.

discovery_cli: announce <name> <tcp_port> [flags] — prints found/lost events.

Also notes future config module in planning.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/discovery/Makefile

@@ -0,0 +1,17 @@
+ROOT := $(abspath ../../..)
+include $(ROOT)/common.mk
+
+MODULE_BUILD = $(BUILD)/discovery
+
+.PHONY: all clean
+
+all: $(MODULE_BUILD)/discovery.o
+
+$(MODULE_BUILD)/discovery.o: discovery.c $(ROOT)/include/discovery.h | $(MODULE_BUILD)
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+$(MODULE_BUILD):
+	mkdir -p $@
+
+clean:
+	rm -f $(MODULE_BUILD)/discovery.o