Module wasm

@causl/causl-wasm-ts/wasm — opt-in WASM backend entry point.

This subpath is the lazy-load seam for the WebAssembly-backed engine described in EPIC #680. Importing @causl/core does not pull this module in; only callers who explicitly import('@causl/causl-wasm-ts/wasm') (or who flip createCausl({ backend: 'wasm' | 'auto' }) past the auto-adapt threshold once #686 lands) pay the WASM bundle cost.

The cost shape:

• @causl/core main bundle: tiny loader stub (~1 KB).
• @causl/causl-wasm-ts/wasm: ~60–120 KB compressed WASM module + ~5 KB of JS bindings, fetched on demand.

For this skeleton (PR #684), the WASM artifacts produced by #682

• #683 + #693 are not yet wired through. loadWasmBackend() therefore resolves a placeholder bridge identifier; the concrete instantiate() path is filled in once those sub-tasks land. The exported types (BackendEngine, Bridge, BridgeFeatures, WasmBackendOptions) are the public-API contract this entry point commits to today and the later sub-tasks must respect.