Standardize unimplemented API failures

RUSTREWRITE.md

@@ -254,6 +254,27 @@ must have their real values. Never return plausible defaults such as `false`,
 zero, an empty collection, or a nil UUID: those could make translated tests pass
 for the wrong reason.
 
+The failure contract lives in `libremetaverse-types` and every API crate exposes
+it as `crate::Error`:
+
+- synchronous fallible members return `Err(NotImplemented::new(CSHARP_ID))`;
+  async members do the same when first polled and start no work first;
+- a stream-producing operation returns the typed error before exposing a
+  stream; an intrinsically infallible stream factory uses `unimplemented_api!`
+  instead of returning an empty stream;
+- callback registration and stored-trait construction fail before retaining the
+  callback/object; generated callbacks are never silent no-ops;
+- infallible constructors, accessors, operators, and trait methods call
+  `unimplemented_api!(CSHARP_ID)`, whose panic marker is distinct from every
+  typed domain error;
+- constants, enum discriminants, flags, and inert public fields use their real
+  catalog values/layout. Generated shims do not derive `Default` unless the
+  reviewed API explicitly defines that behavior.
+
+`python3 tools/generate_rust_mapping.py --check` scans every generated shim and
+rejects `Default` derives, plausible false/zero/empty/nil returns, and any
+function body that lacks the standardized typed error or panic mechanism.
+
 The signature gate is complete only when:
 
 - the metadata catalog reports 100% public type/member mapping coverage for all