feat: seal peer sync with X25519 + XChaCha20-Poly1305 (0.9.17)

Encrypt parliament context sync after Realm mutual auth without pulling in OpenSSL.

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

include/cst_peer.h

@@ -8,17 +8,22 @@
 extern "C" {
 #endif
 
-/* Mutual Realm certificate challenge over TCP (cleartext for now; TLS wrap
- * is the next slice). Same Realm public key required on both sides. */
+/* Mutual Realm certificate challenge over TCP, then ephemeral X25519 +
+ * XChaCha20-Poly1305 (Monocypher) for the session. Same Realm public key
+ * required on both sides. */
 
 typedef struct {
   cst_realm_cert_t remote;
   char host[64];
   int port;
   intptr_t sock; /* open until cst_peer_close */
+  uint8_t aead_key[32];
+  uint64_t send_ctr;
+  uint64_t recv_ctr;
+  int sealed; /* 1 after key exchange */
 } cst_peer_session_t;
 
-/* Dial host:port, prove our cert, verify theirs. 0 on success.
+/* Dial host:port, prove our cert, verify theirs, establish AEAD session.
  * Leaves sock open for post-handshake sync; call cst_peer_close. */
 int cst_peer_dial(const char *host, int port, const cst_realm_cert_t *self,
                   const uint8_t self_sk[64], const uint8_t realm_pk[32],
@@ -33,8 +38,8 @@ int cst_peer_accept(intptr_t listen_sock, const cst_realm_cert_t *self,
 
 void cst_peer_close(cst_peer_session_t *s);
 
-/* After handshake: exchange parliament revision/context. Higher revision wins
- * and is written locally via the callbacks. Returns 0 on success. */
+/* After handshake: exchange parliament revision/context over the sealed
+ * channel. Higher revision wins. Returns 0 on success. */
 typedef struct {
   unsigned long revision;
   char parliament_id[64];

include/cst_version.h

@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 9
-#define CST_VERSION_PATCH 16
+#define CST_VERSION_PATCH 17
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)