spec: incorporate MeshCore discussion #2538 review feedback

Incorporates two of the three corrections raised by msmuenchen on
discussion #2538:

- Identity: document that the 64-byte Ed25519 private key is the
  expanded `a || RH` (orlp) form, not a 32-byte seed, and that it
  cannot be consumed by standard Ed25519 libraries. Touches
  15-identity.md and 14-crypto.md.
- Group: document the GRP_TXT/GRP_DATA inner plaintext format
  (timestamp + flags + message) and the GRP_TXT length limits in
  08-payload-group.md.

The third correction (GRP_DATA vs RAW_CUSTOM app differentiation) is
deferred pending reviewer clarification.

Corpus: replace the unrealistic group vectors with valid inner-format
plaintexts; the ed25519 vectors now carry the 64-byte expanded key and
the seed; the schema and validator gain seed fields. Also corrects two
pre-existing wrong values found while verifying -- ed-001's public key
and ed-002's signature now match RFC 8032.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

corpus/schema/test-vector.schema.json

@@ -329,17 +329,25 @@
       "type": "object",
       "additionalProperties": false,
       "properties": {
+        "sender_seed": {
+          "$ref": "#/$defs/hex_string",
+          "description": "32-byte Ed25519 seed of sender; SHA-512-expanded and clamped to form the 64-byte private key"
+        },
         "sender_private_key": {
           "$ref": "#/$defs/hex_string",
-          "description": "64-byte Ed25519 private key of sender"
+          "description": "64-byte expanded Ed25519 private key of sender (clamped scalar a || nonce prefix RH)"
         },
         "sender_public_key": {
           "$ref": "#/$defs/hex_string",
           "description": "32-byte Ed25519 public key of sender"
         },
+        "recipient_seed": {
+          "$ref": "#/$defs/hex_string",
+          "description": "32-byte Ed25519 seed of recipient; SHA-512-expanded and clamped to form the 64-byte private key"
+        },
         "recipient_private_key": {
           "$ref": "#/$defs/hex_string",
-          "description": "64-byte Ed25519 private key of recipient"
+          "description": "64-byte expanded Ed25519 private key of recipient (clamped scalar a || nonce prefix RH)"
         },
         "recipient_public_key": {
           "$ref": "#/$defs/hex_string",