BridgeJS: Support optional @JSClass as exported function parameters

[krodak]

Overview

Fixes #751. BridgeJS code generation failed to compile when a @JS exported function took (or returned) an optional @JSClass parameter such as JSAbortSignal?, producing no exact matches in call to static method 'bridgeJSLiftParameter' for Optional<JSAbortSignal>.

Root cause

#738 moved optional jsObject to the stack ABI and updated the JS glue, the ImportTS side, and the runtime lowering. However the export parameter direction in ExportSwift was never updated: liftParameterInfo() for .nullable(.jsObject) still emitted direct ABI params and the lift call Optional<T>.bridgeJSLiftParameter(isSome, value).

• For @JSClass types there is no such two-argument overload, so generated wasm thunks failed to compile.
• For plain JSObject? it compiled, but silently mismatched the JS side, which now pushes the presence flag and object id onto the bridge stack. This went unnoticed because the only adjacent test had an empty body and asserted doesNotThrow without reading the value back.

Changes

1. ExportSwift: optional jsObject export params use the stack ABI. .nullable(.jsObject) (including @JSClass) now reports isStackUsingParameter and produces no direct ABI parameters, so the thunk lifts via the no-argument Optional<T>.bridgeJSLiftParameter() that pops the presence flag and object id off the bridge stack, matching the JS lowering.

2. Runtime: add Optional<_JSBridgedClass>.bridgeJSLowerReturn(). @JSClass wrappers already conform to _BridgedSwiftStackType, so lifting and stack push/pop were already available via the generic extension; only the export return path needed a dedicated lowering.

Before:

let ret = roundTripOptionalImportedClass(v: Optional<Foo>.bridgeJSLiftParameter(vIsSome, vValue))

After:

let ret = roundTripOptionalImportedClass(v: Optional<Foo>.bridgeJSLiftParameter())

Tests

• New end-to-end runtime test exercising an optional imported @JSClass directly as an exported function parameter and return value, asserting the round-tripped value for both the present and null cases (the previous coverage only checked import-direction members via text snapshots, which are never compiled for wasm32).
• Added export-direction snapshot coverage for optional JSObject? and optional @JSClass?.
• Regenerated in-package snapshots and the AoT-committed bindings. Snapshot suites pass across the swift-syntax versions in the CI matrix, the BridgeJS TypeScript declaration check passes, and the full wasm test suite passes.