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/fe.h

@@ -0,0 +1,41 @@
+#ifndef FE_H
+#define FE_H
+
+#include "fixedint.h"
+
+
+/*
+    fe means field element.
+    Here the field is \Z/(2^255-19).
+    An element t, entries t[0]...t[9], represents the integer
+    t[0]+2^26 t[1]+2^51 t[2]+2^77 t[3]+2^102 t[4]+...+2^230 t[9].
+    Bounds on each t[i] vary depending on context.
+*/
+
+
+typedef int32_t fe[10];
+
+
+void fe_0(fe h);
+void fe_1(fe h);
+
+void fe_frombytes(fe h, const unsigned char *s);
+void fe_tobytes(unsigned char *s, const fe h);
+
+void fe_copy(fe h, const fe f);
+int fe_isnegative(const fe f);
+int fe_isnonzero(const fe f);
+void fe_cmov(fe f, const fe g, unsigned int b);
+void fe_cswap(fe f, fe g, unsigned int b);
+
+void fe_neg(fe h, const fe f);
+void fe_add(fe h, const fe f, const fe g);
+void fe_invert(fe out, const fe z);
+void fe_sq(fe h, const fe f);
+void fe_sq2(fe h, const fe f);
+void fe_mul(fe h, const fe f, const fe g);
+void fe_mul121666(fe h, fe f);
+void fe_pow22523(fe out, const fe z);
+void fe_sub(fe h, const fe f, const fe g);
+
+#endif