BridgeJS: Unify @JS struct parameter lowering onto the stack ABI

[kateinoigakukun]

Summary

@JS struct bridgeJSLowerParameter() used toJSObject() (object-id ABI). Swift→JS callbacks and the rest of the stack ABI expect a stack push + JS structHelpers.*.lift(). The object-id path pushed then lifted into a JS object, discarded the id, and left the stacks empty for the callback glue — so JS read garbage.

This matches arrays/dictionaries: lower = stack push. ImportTS non-optional structs use the same stack ABI (optional structs already did).

Problematic case

@JS struct Vector2D {
    var dx: Double
    var dy: Double
}

@JS class TextProcessor {
    @JS func observeVector(_ callback: (Vector2D) -> Void) {
        callback(Vector2D(dx: 1.5, dy: 2.5))
    }
}

processor.observeVector((vector) => {
  // Before: { dx: undefined, dy: undefined, ... }
  // After:  { dx: 1.5, dy: 2.5, ... }
});

Same failure mode for any Swift→JS typed closure that takes a non-optional @JS struct (e.g. observation payloads). Bool fields were especially misleading: empty-stack i32 pops yielded undefined, and undefined !== 0 became true.

Fix

• _BridgedSwiftStruct.bridgeJSLowerParameter() → bridgeJSStackPush()
• ImportTS non-optional struct param/return ABI → stack (same as ExportSwift / optionals)
• Runtime regression: observeVector / mapVector in ClosureSupportTests

Test plan

• BridgeJS plugin tests / snapshots
• ClosureSupportTests.testRunJsClosureSupportTests (struct callback params)
• SwiftStructTests.testSwiftStructInImportedSignature (ImportTS stack unify)