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/sc.h

@@ -0,0 +1,12 @@
+#ifndef SC_H
+#define SC_H
+
+/*
+The set of scalars is \Z/l
+where l = 2^252 + 27742317777372353535851937790883648493.
+*/
+
+void sc_reduce(unsigned char *s);
+void sc_muladd(unsigned char *s, const unsigned char *a, const unsigned char *b, const unsigned char *c);
+
+#endif