P2.6: Verify error mapping implementation already complete

All miroir_* error codes from plan §5 are implemented in
crates/miroir-core/src/api_error.rs with tests passing.

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

notes/miroir-9dj.6.md

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+# P2.6 Error Mapping Verification
+
+## Task
+Implement the error response shape from plan §5 and all `miroir_*` error codes.
+
+## Finding
+The implementation was already complete in `crates/miroir-core/src/api_error.rs`.
+
+## Verification
+
+### Required Error Codes (all present)
+- `miroir_primary_key_required` ✓
+- `miroir_no_quorum` ✓
+- `miroir_shard_unavailable` ✓
+- `miroir_reserved_field` ✓
+- `miroir_idempotency_key_reused` ✓
+- `miroir_settings_version_stale` ✓
+- `miroir_multi_alias_not_writable` ✓
+- `miroir_jwt_invalid` ✓
+- `miroir_jwt_scope_denied` ✓
+- `miroir_invalid_auth` ✓
+
+### HTTP Status Code Mappings (matches plan §5)
+- 400: `PrimaryKeyRequired`, `ReservedField` ✓
+- 401: `JwtInvalid`, `InvalidAuth` ✓
+- 403: `JwtScopeDenied` ✓
+- 409: `IdempotencyKeyReused`, `MultiAliasNotWritable` ✓
+- 503: `NoQuorum`, `ShardUnavailable`, `SettingsVersionStale` ✓
+
+### Tests
+All 23 api_error tests pass:
+- Per-code JSON shape tests for each miroir_* code
+- Meilisearch-native error forwarding (preserved verbatim)
+- HTTP status code mapping verification
+- Round-trip serialization
+- Code string round-trip
+
+### Error Shape
+```json
+{"message": "...", "code": "...", "type": "invalid_request|auth|internal|system", "link": "..."}
+```
+
+Matches Meilisearch format exactly.