BridgeJS: Move usesSideChannelForOptionalReturn and usesStackLifting into descriptor, unify return-type switches

BridgeJS: Code review polish — descriptor-driven liftExpression/emitOptionalPlaceholders, trailing comma fix, throw on unknown types
BridgeJS: Use descriptor-driven handling in JSGlueGen to eliminate per-type switches
BridgeJS: Delete dead code and fix nullable descriptor transparency in catch blocks

Author: krodak

Plugins/BridgeJS/Sources/BridgeJSCore/ClosureCodegen.swift

@@ -148,14 +148,12 @@ public struct ClosureCodegen {
         let abiReturnWasmType: WasmCoreType?
         if signature.returnType == .void {
             abiReturnWasmType = nil
-        } else if let wasmType = try signature.returnType.loweringReturnInfo().returnType {
+        } else if let wasmType = try signature.returnType.loweringReturnInfo() {
             abiReturnWasmType = wasmType
         } else {
             abiReturnWasmType = nil
         }
 
-        let throwReturn = abiReturnWasmType?.swiftReturnPlaceholderStmt ?? "return"
-
         // Build signature using SwiftSignatureBuilder
         let funcSignature = SwiftSignatureBuilder.buildABIFunctionSignature(
             abiParameters: abiParams,

Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift

@@ -1535,15 +1535,15 @@ public struct BridgeJSLink {
 
         switch enumDefinition.enumType {
         case .simple:
-            let fragment = IntrinsicJSFragment.simpleEnumHelper(enumDefinition: enumDefinition)
+            let fragment = IntrinsicJSFragment.caseEnumHelper(enumDefinition: enumDefinition)
             _ = try fragment.printCode([enumValuesName], context)
             jsTopLevelLines.append(contentsOf: printer.lines)
         case .rawValue:
             guard enumDefinition.rawType != nil else {
                 throw BridgeJSLinkError(message: "Raw value enum \(enumDefinition.name) is missing rawType")
             }
 
-            let fragment = IntrinsicJSFragment.rawValueEnumHelper(enumDefinition: enumDefinition)
+            let fragment = IntrinsicJSFragment.caseEnumHelper(enumDefinition: enumDefinition)
             _ = try fragment.printCode([enumValuesName], context)
             jsTopLevelLines.append(contentsOf: printer.lines)
         case .associatedValue:
@@ -2172,7 +2172,7 @@ extension BridgeJSLink {
                 printer.write("} catch (error) {")
                 printer.indent {
                     printer.write("setException(error);")
-                    if let abiReturnType = returnType.abiReturnType {
+                    if !returnType.isOptional, let abiReturnType = returnType.descriptor.wasmReturnType {
                         printer.write("return \(abiReturnType.placeholderValue)")
                     }
                 }