C# bridge ABI and wire contract

The managed runtime imports one native symbol: baml_get_api_v1. It validates the returned table's ABI version, required prefix size, and every required function pointer before use. New optional functions are appended to BamlApiV1 and gated by struct_size; existing fields and offsets are never reordered. Buffers and opaque handles cross the boundary with explicit clone, release, and free operations. Tests cover truncated prefixes, missing entries, callback and cancellation lifetimes, buffer ownership, media ownership, and exact handle release.

The unified call_function(encoded_args, length, callback_id) signature is function-table ABI revision 2. Revision-1 hosts and runtimes reject revision-2 counterparts before reading the changed function-pointer slot, preventing mixed-revision calls from invoking incompatible layouts.

The canonical protobuf schemas live in crates/bridge_ctypes/types/baml_bridge/cffi/v1. Schema evolution is additive: field numbers are stable, new metadata is optional on the wire, and decoders must accept legacy messages that omit it. Present but malformed collection or union metadata fails closed. The C# runtime consumes these schemas directly with build-time Grpc.Tools; it does not maintain a private schema copy.

Generated C# types carry canonical serialized BamlTy metadata through names, union cases, descriptors, registry factories, and codecs. The C# generator normalizes unions once at its boundary so all those layers observe the same ordering. Maps accept only BAML string keys.

Generated enum values use the stable baml-csharp-enum-discriminant-v1 identity grammar. Each field is encoded as a one-byte tag, a big-endian unsigned 32-bit UTF-8 byte length, and the UTF-8 bytes. Counts use a one-byte tag followed by a big-endian unsigned 32-bit count. The ordered input is:

1. identity field 0x00 containing the grammar name;
2. package count 0x10, followed by package field 0x11 when the package is not the implicit user package;
3. namespace count 0x20, followed by one namespace field 0x21 per segment;
4. enum symbol field 0x30 and variant symbol field 0x31.

The signed discriminant is the first eight SHA-256 bytes interpreted as a big-endian unsigned integer with the high bit cleared. Generation rejects zero and collisions within an enum.

Opaque resource values are emitted only for classes whose compiled metadata uses the tagged heap-handle representation. Inbound handles must belong to the active heap, still resolve, match their declared type arguments, and resolve to an instance of a tagged resource class. User classes remain structural.