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/license.txt

@@ -0,0 +1,16 @@
+Copyright (c) 2015 Orson Peters <orsonpeters@gmail.com>
+
+This software is provided 'as-is', without any express or implied warranty. In no event will the
+authors be held liable for any damages arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose, including commercial
+applications, and to alter it and redistribute it freely, subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not claim that you wrote the
+   original software. If you use this software in a product, an acknowledgment in the product
+   documentation would be appreciated but is not required.
+
+2. Altered source versions must be plainly marked as such, and must not be misrepresented as
+   being the original software.
+
+3. This notice may not be removed or altered from any source distribution.