feat: implement RNS transport (Smol Mail 1.2) over microReticulum

shim/smolmail_rns.h

@@ -0,0 +1,55 @@
+/*
+ * C ABI between bunshin (Rust) and the microReticulum carrier shim.
+ *
+ * The shim owns the Reticulum run loop on a dedicated thread; every callback
+ * below fires on that thread. link_id is always 16 bytes.
+ */
+#ifndef SMOLMAIL_RNS_H
+#define SMOLMAIL_RNS_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Implemented in smolmail_rns.cpp, called from src/rns.rs.
+ * identity: 64 bytes, x25519 private (32) || ed25519 private (32), the same
+ * layout Identity::to_file writes and load_private_key expects.
+ * udp_forward_host may be NULL: with no forward target the interface sends
+ * to the source address of the last datagram received.
+ * Returns 0 on success, negative on failure. */
+int smolmail_rns_start(const uint8_t *identity,
+                       const char *storage_dir,
+                       const char *udp_listen_host, uint16_t udp_listen_port,
+                       const char *udp_forward_host, uint16_t udp_forward_port,
+                       uint8_t *destination_hash_out);
+
+/* Pure computation, no Reticulum state: the 16-byte destination hash of
+ * smolmail.server under this identity. microReticulum derives the identity's
+ * x25519 public key with an unclamped scalar multiplication, which differs
+ * from RFC 7748, so the hash must come from the library itself rather than
+ * an independent Rust derivation. */
+int smolmail_rns_destination_hash(const uint8_t *identity,
+                                  uint8_t *destination_hash_out);
+
+/* Implemented in src/rns.rs, called from the shim on the Reticulum loop
+ * thread. A request is `op u8 || body` (RNS.md sec 1); the response placed
+ * into the slot is `status u8 || payload`. on_request returns the response
+ * length and leaves the bytes in the slot; the shim must copy them out with
+ * take_response before the next on_request call. */
+size_t smolmail_rns_on_request(const uint8_t *request, size_t request_len,
+                               const uint8_t *link_id);
+size_t smolmail_rns_take_response(uint8_t *out, size_t cap);
+
+/* 0 = link admitted, 1 = over the concurrent-link cap (the shim tears the
+ * link down after the current loop iteration). */
+int smolmail_rns_on_link_opened(const uint8_t *link_id);
+void smolmail_rns_on_link_closed(const uint8_t *link_id);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SMOLMAIL_RNS_H */