refactor: structured body (array|object, never string) for canonicalization

Message bodies are always arrays of strings (text lines) or objects
(structured data like PUBKEY). Never raw strings. This enables:
- Multiline messages without escape sequences
- Deterministic JSON canonicalization (RFC 8785 JCS) for signing
- Structured data where needed

Update all schemas: body fields use array type with string items.
Update message.json envelope: body is oneOf[array, object], id is UUID.
Update README: message envelope table, examples, and canonicalization docs.
Update schema/README.md: field types, examples with array bodies.

schema/message.json

@@ -6,8 +6,9 @@
   "type": "object",
   "properties": {
     "id": {
-      "type": "integer",
-      "description": "Server-assigned message ID, monotonically increasing. Present on all server-originated messages."
+      "type": "string",
+      "format": "uuid",
+      "description": "Server-assigned message UUID. Present on all server-originated messages."
     },
     "command": {
       "type": "string",
@@ -28,8 +29,19 @@
       "description": "Additional parameters (used primarily by numeric replies). Equivalent to IRC middle parameters."
     },
     "body": {
-      "type": "string",
-      "description": "Message body / trailing parameter. Equivalent to IRC trailing parameter (after the colon)."
+      "oneOf": [
+        {
+          "type": "array",
+          "items": { "type": "string" },
+          "description": "Array of strings (one per line for text messages)."
+        },
+        {
+          "type": "object",
+          "description": "Structured data (e.g. PUBKEY key material).",
+          "additionalProperties": true
+        }
+      ],
+      "description": "Message body. MUST be an array or object, never a raw string. Arrays represent lines of text; objects carry structured data. This enables deterministic canonicalization (RFC 8785 JCS) for signing."
     },
     "ts": {
       "type": "string",
@@ -38,7 +50,21 @@
     },
     "meta": {
       "type": "object",
-      "description": "Extensible metadata (signatures, rich content hints, etc.). Not present in original IRC.",
+      "description": "Extensible metadata. Used for signatures (meta.sig, meta.alg), hashes (meta.hash), and client extensions.",
+      "properties": {
+        "sig": {
+          "type": "string",
+          "description": "Base64-encoded cryptographic signature over the canonical message form."
+        },
+        "alg": {
+          "type": "string",
+          "description": "Signature algorithm (e.g. 'ed25519')."
+        },
+        "hash": {
+          "type": "string",
+          "description": "Hash of the canonical message form (e.g. 'sha256:base64...')."
+        }
+      },
       "additionalProperties": true
     }
   },