NFC: BridgeJS: Refactor JSGlueGen with compositional optional handling and helper consolidation

[krodak]

Mix of concrete simplifications and structural refactoring in JSGlueGen.swift. Happy to split this into a smaller PR with just the simplification items if the refactoring part is too much for one review.

Simplifications

• Factory collapse: Struct and enum helper factories collapsed from () => { return () => ({...}); } to () => ({...}) - the double-invocation wrapper was left over from PR NFC: BridgeJS: Remove dead cleanup infrastructure #655
• simpleEnumHelper/rawValueEnumHelper unified into caseEnumHelper - they had identical logic
• Dead code removed: emitPushI32Return, emitPushF64Return, emitPushPointerReturn were copies of their parameter counterparts; several fragment aliases that just forwarded to another fragment
• emitRetainCase helper extracted to deduplicate 5 identical JSValue retain-and-break cases
• jsZeroLiteral scoped down from public WasmCoreType API in BridgeJSSkeleton to fileprivate in JSGlueGen - nobody else uses it
• Redundant (ret | 0) coercion dropped from optional case enum returns - the non-optional path never applied it, and the value is already an integer from the enum helper so | 0 was a no-op

Structural refactoring

• Compositional optional handling: Instead of separate IntrinsicJSFragment definitions for every optional(X) combination, generic fragments wrap the inner type's fragments. Adding a new BridgeType no longer requires writing dedicated optional fragment variants - you set a few properties and optional support composes from the base fragments. The stack ABI optional path is fully generic, which should help with planned external class support.
• Type-knowledge properties on BridgeType: wasmParams, optionalConvention, nilSentinel, isSingleParamScalar, lowerCoerce, etc. moved from free functions and static methods into a private extension BridgeType in JSGlueGen.swift. Reads as type.wasmParams instead of wasmParams(for: type).
• reserveNames scope change to fileprivate, moved to bottom of JSGlueVariableScope

Trade-offs

The composite optional wrapper produces slightly more verbose JS than the old per-type fragments (result intermediary vars and if/else vs inline ternaries). The codegen is easier to extend and engines optimize both patterns the same way.

[kateinoigakukun]

Nice!

[kateinoigakukun]

arguments given to IntrinsicJSFragment are not guaranteed to be a variable name but it can be an arbitrary expression, so we can't register them into the scope.

Instead, what we really need to do here seems like registering parameter names into the scope?

class ExportedThunkBuilder {
    func lowerParameter(param: Parameter) throws {
        let loweringFragment = try IntrinsicJSFragment.lowerParameter(type: param.type)
        assert(
            loweringFragment.parameters.count == 1,
            "Lowering fragment should have exactly one parameter to lower"
        )
        let paramName = scope.variable(param.name)
        self.parameterNames.append(paramName)
        let loweredValues = try loweringFragment.printCode([paramName], context)
        parameterForwardings.append(contentsOf: loweredValues)
    }
}

[kateinoigakukun]

Nice cleanup!