feat(firmware): MeshCore message RX — decrypt DMs addressed to us

- vendor orlp/ed25519 (lib/ed25519, MIT — the exact lib MeshCore uses) for
  Curve25519 ECDH key exchange; switch identity/advert signing to it (same
  seed -> same pubkey, so peers still recognise us; sigs are byte-identical)
- store pubkeys from heard MeshCore adverts (contacts table) so a DM's
  1-byte src_hash can be resolved to a full sender key
- decode PAYLOAD_TYPE_TXT_MSG: ECDH shared secret, HMAC-SHA256 MAC check,
  AES-128-ECB decrypt, parse timestamp+text, drive the message modal
- verified end-to-end: real MeshCore node -> "hello from HP Pro Desk" decoded

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

firmware/lib/ed25519/sha512.h

@@ -0,0 +1,21 @@
+#ifndef SHA512_H
+#define SHA512_H
+
+#include <stddef.h>
+
+#include "fixedint.h"
+
+/* state */
+typedef struct sha512_context_ {
+    uint64_t  length, state[8];
+    size_t curlen;
+    unsigned char buf[128];
+} sha512_context;
+
+
+int sha512_init(sha512_context * md);
+int sha512_final(sha512_context * md, unsigned char *out);
+int sha512_update(sha512_context * md, const unsigned char *in, size_t inlen);
+int sha512(const unsigned char *message, size_t message_len, unsigned char *out);
+
+#endif