WASM decider: typed cross-language codec and proto to WIT emitter

[yordis]

From the WASM decider roadmap (formerly WASM_TODO.md, now retired).

Publish a per-stream-type codec library component (encode/decode of version-faithful typed events) with WIT types generated from the same proto source, as an Nth codegen target beside Rust and Elixir.

Requirements:

• Map proto open enums to WIT closed variants with an explicit unknown case
• Handle field presence, maps, well-known types, and recursion
• Until this lands, cross-language consumers decode event bytes with their own native proto library

Why: cross-language consumers of decider event streams currently need their own proto toolchain; a shared codec component removes that duplication and keeps event typing version-faithful across hosts.

[yordis]

Sequencing note, so this is not blocked on something that has already happened.

This was held behind the module-descriptor contract decision, which would have collided with it. That decision landed in the trogon:decider@0.3.0 bump (command-spec gained a required snapshot-policy, and write-precondition dropped its option<> wrapper), so the constraint is discharged and nothing in the decider stack gates this issue any more.

Scope is unchanged and still project-sized rather than task-sized: an Nth codegen target beside Rust and Elixir, a published per-stream-type codec component, and a proto-to-WIT emitter that has to answer open enums (closed WIT variants with an explicit unknown case), field presence, maps, well-known types, and recursion.

Until it ships, cross-language consumers decode event bytes with their own proto library. That is the documented current answer, not a regression.