BridgeId:
    | "wasmgc-builtins"
    | "wasmgc-classic"
    | string & { __brand?: "FutureBridge" }

Stable identifier for the shipped bridge plus a forward-compatible escape hatch for future bridges (shared-memory, string-view, …) that land against the same interface. The legacy serde-json bridge was retired with the split-crate engine_rs lineage (issue #10); the consolidated causl_engine_bridge crate now produces ONE WasmGC feature variant, wasmgc-classic.

'wasmgc-builtins' stays a member of the union DELIBERATELY, and it is not a bridge a host may select. causl/causl-core-rs#355 retired it and the loader refuses it at selection time; keeping the literal is what lets preloadCauslWasm({ bridge: 'wasmgc-builtins' }) type-check and then fail LOUD with the measurement, instead of collapsing into the FutureBridge escape hatch and reading as an unknown id. See detectBridge for the retirement and its way back.