WIT-driven IPC topic schemas for A2UI #643

Problem

IPC bus payloads are opaque JSON strings. The runtime has no schema metadata for topic payloads, so the LLM (A2UI bridge) cannot know what fields a topic carries or what they mean. New capsules are invisible to the UI.

Solution

Let capsule authors define WIT records for their IPC event types. At install time, wit-parser extracts field names, types, and /// doc comments into JSON Schema. The schema is baked into meta.json and fed to the SchemaCatalog at runtime.

Capsule author workflow

1. Write wit/events.wit with ///-documented records
2. Declare [[topic]] in Capsule.toml with wit_type = "record-name"
3. Use the generated Rust struct in capsule code via ipc::publish_json()
4. Install → runtime populates schema catalog → LLM sees typed schema

Changes

• wit-parser 0.227 workspace dependency
• astrid-build::wit_schema module (WIT record → JSON Schema converter)
• wit_type: Option<String> field on TopicDef
• bake_topics() extended for WIT type resolution
• SchemaCatalog::register_topics() accepts baked schemas
• WasmEngine::load() reads meta.json and populates schema catalog