parameter-metadata: emit the SDK JSON documents with serde_json and generate the worklet ABI constants (deferred past launch)

[tamashi095]

Child of #377 (Part 2, finding 3, plus finding 8). Deferred past launch by the orchestrator's judgement on 2026-09-04: it re-pins two shipped SDK JSON assets and the generated TypeScript a week after @misofm/engine-sdk 0.1.0 shipped, for a moderate robustness gain. Owner may pull it forward.

Scope

• Replace the ~360 lines of push_str/format! JSON assembly in tools/parameter-metadata/src/lib.rs:129-273 and src/abi_layout.rs:~435-650 with #[derive(Serialize)] models and serde_json (preserve_order, already in Cargo.lock as a dev-dependency elsewhere). Decide float spelling: serde_json uses ryu for f32, the current number() uses {:?}; both are shortest-round-trip but exponent thresholds differ, so either accept a one-time re-pin or emit numbers through RawValue.
• While there, emit a constants prelude from abi_layout.rs so the hand-typed COMMAND_RECORD_BYTES = 48 in hosts/host-web/web/miso-engine-v1-audio-worklet.js:20 and -host.js:51 come from the generator (finding 8), or add them to check-session-map-shape.py's equality set.
• Re-pin sdk/assets/*.json, sdk/src/generated/{catalog,abi}.ts, and run check-parameter-metadata-v1.py, check-abi-layout-v1.py --self-test, check-sdk-generated.sh, check-web-audioworklet.sh.

Not scheduled

Do not claim before launch without an owner comment here.