SpiderMonkey-backed API compatibility layer replacing bun_jsc.

bun_sm provides the same public API surface as the phantom bun_jsc crate, but backed by SpiderMonkey via bao_engine + mozjs. When bun_runtime changes pub use bun_jsc::* to pub use bun_sm::*, all 8,500+ JSC references resolve to SM-backed implementations.

Architecture

JSC types map to SM equivalents:

• JSGlobalObject → *mut JSContext (SM's JSContext owns the heap)
• JSValue → newtype over bun_sm::value::JsValue
• CallFrame → wrapper over SM CallArgs
• VirtualMachine → TLS singleton wrapping BaoEventLoop
• JSPromise → ZST wrapping *mut JSObject

ABI difference

JSC: unsafe extern "C" fn(*mut JSGlobalObject, *mut CallFrame) -> JSValue
SM:  unsafe extern "C" fn(*mut JSContext, argc: u32, vp: *mut JSVal) -> bool