feat: same-Realm peer discovery via UDP and local registry (0.9.10)

Advertise instances with a multicast beacon and a config-dir registry so
same-machine and LAN peers that share a Realm fingerprint can find each
other without TOFU pairing.

Co-authored-by: Cursor <cursoragent@cursor.com>

include/cst_discover.h

@@ -0,0 +1,46 @@
+#ifndef CST_DISCOVER_H
+#define CST_DISCOVER_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Same-Realm peer discovery: UDP multicast beacon + local instance file.
+ * mDNS (_cst-parliament._tcp) can replace the beacon later without changing
+ * this surface. */
+
+#define CST_DISCOVER_PORT 45454
+#define CST_DISCOVER_GROUP "239.255.77.77"
+
+typedef struct {
+  char instance_id[64];
+  char realm_fp[33];
+  char name[64];
+  char host[64];
+  int port;
+} cst_peer_info_t;
+
+typedef struct cst_discover cst_discover_t;
+
+/* Advertise this instance and collect peers that share realm_fp.
+ * peer_port is the TCP parliament port peers should dial. */
+cst_discover_t *cst_discover_start(const char *realm_fp, const char *instance_id,
+                                   const char *name, int peer_port);
+void cst_discover_stop(cst_discover_t *d);
+
+/* Copy up to max known peers (excluding self). Returns count. */
+int cst_discover_peers(cst_discover_t *d, cst_peer_info_t *out, int max);
+
+/* Same-machine registry under <config>/realm/local-instances.json. */
+int cst_discover_local_put(const char *realm_fp, const char *instance_id,
+                           const char *name, int peer_port);
+int cst_discover_local_del(const char *instance_id);
+int cst_discover_local_list(const char *realm_fp, cst_peer_info_t *out, int max);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

include/cst_version.h

@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 9
-#define CST_VERSION_PATCH 9
+#define CST_VERSION_PATCH 10
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)