refactor(crypto): replace orlp/ed25519 with Monocypher 4.0.2

Swap the vendored orlp/ed25519 (frozen ~2017 ref10) for Monocypher
4.0.2, an actively maintained, audited, single-file implementation.

The persisted private key keeps its 64-byte expanded layout
(clamped SHA-512(seed) scalar a || nonce prefix), so identities
written by older firmware load, sign, verify and key-exchange
unchanged -- no re-key, no storage migration, full wire
compatibility with the existing mesh and Arduino MeshCore.

Because the stored key carries no seed for Monocypher's high-level
EdDSA API, Identity drives signing from the low-level primitives
(crypto_sha512 + crypto_eddsa_reduce/scalarbase/mul_add); verify
uses crypto_ed25519_check and ECDH uses crypto_eddsa_to_x25519 +
crypto_x25519. Nonce material is now wiped after signing.

Validated byte-for-byte against the previous orlp output via a
known-answer harness (keygen, sign-from-expanded-key, verify
accept/reject, X25519-over-Ed25519 shared secret) before the swap.

Frees ~42 KB of flash: orlp linked ~55 KB (dominated by its ~30 KB
ref10 precomputed tables); the Monocypher Ed25519/X25519/SHA-512
paths link ~14 KB, with --gc-sections dropping all unused algos.
wio_tracker_l1 pristine build: FLASH 54.14%, links clean.

Monocypher is CC0-1.0 OR BSD-2-Clause.

zephcore/app/CompanionMesh.cpp

@@ -6,7 +6,6 @@
 #include "CompanionMesh.h"
 #include <mesh/Utils.h>
 #include <helpers/TxtDataHelpers.h>
-#include <ed_25519.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>