Define complete Rust API mapping schema

api/README.md

@@ -12,10 +12,13 @@ LibreMetaverse library assemblies at the pinned upstream commit. It contains:
   every LibreMetaverse API that uses them.
 
 External entries are replacement obligations, not copied dependency APIs.
-Issue #3 must map each one to a Rust crate/path: use Rust core/std or an adopted
-crate where it preserves the contract, otherwise define a native MetaCrate
-type. The catalog deliberately does not enumerate members of SkiaSharp,
-SIPSorcery, MessagePack, CoreJ2K, or any other external assembly.
+`RUST-TYPES.tsv` resolves every entry to Rust core/std, an adopted
+cross-platform crate, or a native MetaCrate type. Third-party API objects use
+project-owned boundary types so their eventual backend can be replaced without
+exposing SkiaSharp, SIPSorcery, MessagePack, CoreJ2K, or another foreign member
+surface. `RUST-MAPPING.tsv` records one reviewed destination and full Rust
+signature for every catalog member. `MAPPING-COVERAGE.md` is the readable gate
+summary.
 
 Regenerate from the clean pinned upstream checkout:
 
@@ -28,3 +31,17 @@ extractor twice against the same compiled metadata, rejects count mismatches or
 duplicate documentation IDs, and writes the result only when both outputs are
 byte-identical. The extractor is development-only and is absent from every
 Cargo manifest and published Rust dependency graph.
+
+Regenerate or validate the Rust mapping with only Python's standard library:
+
+```sh
+python3 tools/generate_rust_mapping.py
+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.
+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.