Fix PackageToJS plugin wasm-opt fallback when output file exists

When wasm-opt is not installed, DefaultPackagingSystem falls back to copying
the input Wasm file directly to output. However, FileManager.copyItem
fails if the destination file already exists, while wasm-opt overwrites
existing files.

Changes:
- Remove existing output file before copying in wasm-opt fallback
- Make DefaultPackagingSystem injectable for testing with custom which function
- Add focused tests for DefaultPackagingSystem fallback behavior
- Test both new and existing output file scenarios

Fixes issue where builds fail with 'File exists' error when wasm-opt
is unavailable and output file already exists from previous builds.

Plugins/PackageToJS/Sources/PackageToJS.swift

@@ -293,9 +293,11 @@ extension PackagingSystem {
 final class DefaultPackagingSystem: PackagingSystem {
 
     private let printWarning: (String) -> Void
+    private let which: (String) throws -> URL
 
-    init(printWarning: @escaping (String) -> Void) {
+    init(printWarning: @escaping (String) -> Void, which: @escaping (String) throws -> URL = which(_:)) {
         self.printWarning = printWarning
+        self.which = which
     }
 
     func npmInstall(packageDir: String) throws {
@@ -309,6 +311,10 @@ final class DefaultPackagingSystem: PackagingSystem {
     func wasmOpt(_ arguments: [String], input: String, output: String) throws {
         guard let wasmOpt = try? which("wasm-opt") else {
             _ = warnMissingWasmOpt
+            // Remove existing output file if it exists (to match wasm-opt behavior)
+            if FileManager.default.fileExists(atPath: output) {
+                try FileManager.default.removeItem(atPath: output)
+            }
             try FileManager.default.copyItem(atPath: input, toPath: output)
             return
         }

Plugins/PackageToJS/Tests/DefaultPackagingSystemTests.swift

@@ -0,0 +1,60 @@
+import Foundation
+import Testing
+
+@testable import PackageToJS
+
+@Suite struct DefaultPackagingSystemTests {
+
+    @Test func wasmOptFallbackHandlesNewOutputFile() throws {
+        try withTemporaryDirectory { tempDir, _ in
+            let inputFile = tempDir.appendingPathComponent("input.wasm")
+            let outputFile = tempDir.appendingPathComponent("output.wasm")
+            let inputContent = Data("input wasm content".utf8)
+            try inputContent.write(to: inputFile)
+
+            var warnings: [String] = []
+            // Create system with mock which function that always fails to find wasm-opt
+            let system = DefaultPackagingSystem(
+                printWarning: { warnings.append($0) },
+                which: { _ in throw PackageToJSError("wasm-opt not found") }
+            )
+
+            // This should work - fallback should copy file
+            try system.wasmOpt(["-Os"], input: inputFile.path, output: outputFile.path)
+
+            // Verify the output file was created with input content
+            let finalContent = try Data(contentsOf: outputFile)
+            #expect(finalContent == inputContent)
+            #expect(warnings.contains { $0.contains("wasm-opt is not installed") })
+        }
+    }
+
+    @Test func wasmOptFallbackHandlesExistingOutputFile() throws {
+        try withTemporaryDirectory { tempDir, _ in
+            let inputFile = tempDir.appendingPathComponent("input.wasm")
+            let outputFile = tempDir.appendingPathComponent("output.wasm")
+            let inputContent = Data("input wasm content".utf8)
+            let existingContent = Data("existing output content".utf8)
+
+            // Create input file and existing output file
+            try inputContent.write(to: inputFile)
+            try existingContent.write(to: outputFile)
+
+            var warnings: [String] = []
+            // Create system with mock which function that always fails to find wasm-opt
+            let system = DefaultPackagingSystem(
+                printWarning: { warnings.append($0) },
+                which: { _ in throw PackageToJSError("wasm-opt not found") }
+            )
+
+            // This should work - fallback should overwrite existing file
+            try system.wasmOpt(["-Os"], input: inputFile.path, output: outputFile.path)
+
+            // Verify the output file was overwritten with input content
+            let finalContent = try Data(contentsOf: outputFile)
+            #expect(finalContent == inputContent)
+            #expect(finalContent != existingContent)
+            #expect(warnings.contains { $0.contains("wasm-opt is not installed") })
+        }
+    }
+}