Standardize unimplemented API failures

api/MAPPING-COVERAGE.md

@@ -12,6 +12,7 @@ Generated by `python3 tools/generate_rust_mapping.py`; do not edit by hand.
 - Stale or missing catalog IDs: **0**
 - Unresolved referenced types: **0**
 - Platform-specific mappings: **0**
+- Plausible generated fallback bodies/default derives: **0**
 
 ## Assembly representatives
 

api/README.md

@@ -42,6 +42,9 @@ python3 tools/generate_rust_mapping.py --check
 The checker rejects duplicate C# IDs, duplicate Rust destinations, invalid
 statuses, stale or missing catalog entries, unresolved signature types,
 assemblies without representative mappings, and platform-specific API targets.
+It also rejects generated `Default` shims, plausible false/zero/empty/nil
+fallbacks, and generated function bodies that do not use the standardized
+`NotImplemented` error or `unimplemented_api!` panic.
 The two support traits in `RUST-TYPES.tsv` are included because compiled public
 inheritance records reference them even though they are absent from the public
 type table.