types: add Api, Type, and SourceContext well-known types

[Divyansh151005]

Fixes #382.

What this does

buffa-types only shipped the JSON-mappable well-known types. Codegen still auto-maps the entire .google.protobuf package there, so compiling googleapis (or any schema that imports api.proto / type.proto / source_context.proto) failed with cannot find type Api / TypeView / EnumView.

This vendors those three remaining official well-known files (protobuf v33.5, matching the existing any.proto pin and the tools image) and generates them next to Timestamp/Any:

• Binary + view round-trips in buffa-types
• register_wkt_types textproto Any-expansion for the new messages
• A wkt_api.proto fixture so buffa-test compiles a Catalog with Api / Type / Enum / SourceContext fields
• A codegen integration assertion that those types resolve through ::buffa_types::google::protobuf::{Api,Type,Enum,SourceContext} and the matching views

JSON serde is intentionally not generated. These types use the standard proto3 object mapping (no special encoding), and turning generate_json on for all WKTs would also derive serde for Timestamp/Any and duplicate the hand-written impls. wkt_usage.proto is left unchanged so the json=true compile fixture stays green.

Shipping api.proto also made [Mixin][] resolve in-crate. The generated [Mixin](crate::google::protobuf::Mixin) is a rustdoc::redundant_explicit_links failure under this repo's -D warnings. Same-module AIP-192 refs now emit [Mixin]; FQ display names (google.protobuf.Type) and cross-module destinations still get an explicit crate:: path.

Testing

$ rustc --version
rustc 1.95.0 (59807616e 2026-04-14)

$ cargo fmt --all --check
$ cargo clippy --workspace --all-targets -- -D warnings
$ RUSTDOCFLAGS='-D warnings' cargo doc --workspace --all-features --no-deps
$ cargo test --workspace

All passed. Targeted extras: cargo test -p buffa-types --all-features (including text_registry_roundtrip_source_context), codegen_wkt_api_auto_mapping, and buffa-test test_wkt_api_types_round_trip.

Conformance (task conformance) was not run locally; this change does not touch the codec wire path the suite exercises.

[github-actions]

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[Divyansh151005]

I have read the CLA Document and I hereby sign the CLA

[iainmcgin]

[claude code]

Reviewed. The vendored protos are byte-identical to protobuf v33.5, the codegen config matches the existing WKTs, and the Option shadowing is handled (no bare Option< in the generated tree, not re-exported at the crate root). Pushed 410a465 with the fixups: .github/workflows/ci.yml carried a third copy of the WKT proto list and was the one not updated, which is why check-generated-code failed; the new same-module rustdoc-link rule now keeps the escaped explicit target for a keyword-named type (a bare [type] label would not resolve), with a test; the guide and buffa-types docs say these types have no Serialize/Deserialize and what a json = true consumer sees; the new parts (Method, Mixin, Field, EnumValue, Option) are listed; TypeView/Enum round-trips, Type/Option text-registry round-trips (Option nests an Any), and Catalog.enum_type is populated; the rustdoc-link change moved to its own Changed fragment. Ready to queue once Iain stamps it; thanks for taking #382 on.