Add typed browser WebAssembly adapter

[konojunya]

背景

Checkpoint D requires the engine to expose the same formatter, checker, and renderer behavior to browser consumers without introducing host I/O or a second semantic implementation.

実装方法

• add a stack-engine-wasm adapter that accepts string | Uint8Array and returns typed operation results
• reuse the native engine API for formatting, diagnostics, SVG rendering, and metadata
• publish a browser-focused @stack-sh/engine package layout with explicit license material
• audit generated WebAssembly imports and JavaScript glue for filesystem, network, DOM, and clock capabilities
• document the adapter contract and dependency boundary in ADR 0006
• add Node 20, TypeScript, package-content, browser, and native/WASM parity gates to CI

テスト方法

• cargo +stable test --workspace --locked
• specification conformance suites for formatter and engine
• SVG validation and Wasmtime cross-target numeric fixture
• cargo +stable clippy --workspace --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --workspace --no-deps --locked
• Rust 1.85 workspace tests and clippy
• npm ci && npm run build:wasm && npm test && npm run typecheck && npm run pack:check on Node 20.18
• headless Chromium smoke for format, check, and render
• coverage: 95.52% regions, 97.11% functions, 97.21% lines

Review 視点 / 懸念点

• confirm the JavaScript result shapes are sufficient for the future CLI and web consumers
• confirm the generated package includes exactly the intended runtime and license files
• the synchronous browser API deliberately has no filesystem, network, DOM, or clock capability