docs(pdftract-5xq16): add verification note

Add verification note documenting JSON-RPC 2.0 framing implementation
with all acceptance criteria PASS.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/pdftract-5xq16.md

@@ -0,0 +1,70 @@
+# pdftract-5xq16: JSON-RPC 2.0 framing layer
+
+## Summary
+
+Implemented hand-rolled JSON-RPC 2.0 framing layer at `crates/pdftract-cli/src/mcp/framing/mod.rs`.
+
+## Files Changed
+
+- `crates/pdftract-cli/src/mcp/framing/mod.rs` - New module (658 lines)
+- `crates/pdftract-cli/src/mcp/mod.rs` - Added `pub mod framing;` and re-exports
+
+## Implementation Details
+
+### Types Defined
+
+1. **`Id` enum** - Request identifier preserving JSON type:
+   - `Id::Number(i64)` - Numeric IDs
+   - `Id::String(String)` - String IDs
+   - `Id::Null` - Null IDs (for parse errors)
+
+2. **`Request`** - JSON-RPC request with:
+   - `jsonrpc` field (validated to be "2.0")
+   - `method: String`
+   - `params: Option<Value>`
+   - `id: Option<Id>` (None = notification)
+
+3. **`Notification`** - Request without id field (no response)
+
+4. **`Response`** - Response with mutually exclusive `result` or `error`
+
+5. **`ErrorObject`** - Error with code, message, and optional data
+
+6. **`BatchMessage`** - Single or batch request wrapper
+
+### Error Codes Implemented
+
+| Code | Name | Constructor |
+|------|------|-------------|
+| -32700 | Parse error | `ErrorObject::parse_error()` |
+| -32600 | Invalid Request | `ErrorObject::invalid_request()` |
+| -32601 | Method not found | `ErrorObject::method_not_found(method)` |
+| -32602 | Invalid params | `ErrorObject::invalid_params()` |
+| -32603 | Internal error | `ErrorObject::internal_error()` |
+| -32099..-32000 | Server errors | `ErrorObject::server_error(code, msg)` |
+
+## Acceptance Criteria
+
+All PASS:
+
+- ✅ **Round-trip test**: `test_request_round_trip` - Request serializes and deserializes correctly
+- ✅ **ID preservation**: `test_id_number_preservation`, `test_id_string_preservation`, `test_id_null_preservation` - All 3 ID types preserve JSON type through round-trip
+- ✅ **Parse-error path**: `test_parse_error_response` - Parse error response has `id: null`
+- ✅ **Method-not-found**: `test_method_not_found` - Returns code -32601 with method name in data field
+- ✅ **Notification path**: `test_notification_no_id`, `test_notification_deserialize` - Notifications have no id field; deserialize correctly
+- ✅ **Batch round-trip**: `test_batch_round_trip` - Batch of 3 requests serializes/deserializes correctly
+- ✅ **Error codes**: `test_all_error_codes` - All 6 spec-defined error codes return correct values
+- ✅ **jsonrpc validation**: `test_reject_invalid_jsonrpc_version`, `test_reject_missing_jsonrpc_field` - Invalid versions rejected with clear error
+- ✅ **Empty batch**: `test_empty_batch_rejected` - Empty array returns Invalid Request error
+
+## Test Results
+
+```
+running 16 tests
+........................
+test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured
+```
+
+## Commit
+
+`3c10dfd` - feat(pdftract-5xq16): implement JSON-RPC 2.0 framing layer