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/keypair.c

@@ -0,0 +1,23 @@
+#include "ed_25519.h"
+#include "sha512.h"
+#include "ge.h"
+
+
+void ed25519_create_keypair(unsigned char *public_key, unsigned char *private_key, const unsigned char *seed) {
+    ge_p3 A;
+
+    sha512(seed, 32, private_key);
+    private_key[0] &= 248;
+    private_key[31] &= 63;
+    private_key[31] |= 64;
+
+    ge_scalarmult_base(&A, private_key);
+    ge_p3_tobytes(public_key, &A);
+}
+
+void ed25519_derive_pub(unsigned char *public_key, const unsigned char *private_key) {
+    ge_p3 A;
+
+    ge_scalarmult_base(&A, private_key);
+    ge_p3_tobytes(public_key, &A);
+}