Add authoritative public API catalog

api/README.md

@@ -0,0 +1,30 @@
+# Public API catalog
+
+`public-api.json` is the authoritative compiled-metadata inventory for the 13
+LibreMetaverse library assemblies at the pinned upstream commit. It contains:
+
+- stable C# documentation IDs and complete public type/member metadata;
+- nullability, constraints, bases, interfaces, overload parameters, accessors,
+  constants, events, delegates, enum values, and public attributes;
+- assembly identities and independent reflection/ECMA-335 count checks;
+- hashes for the outputs of all six upstream source generators;
+- `external_types`, the non-LibreMetaverse types used by public signatures and
+  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.
+
+Regenerate from the clean pinned upstream checkout:
+
+```sh
+python3 tools/extract_public_api.py
+```
+
+The command builds the upstream libraries into a temporary directory, runs the
+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.