feat(native): C# input/capture sidecar replacing the PowerShell helper

Implements ADR-008. native/input-helper is a self-contained .NET 9 console exe
speaking the identical JSON-over-stdin/stdout protocol as the old PowerShell
helper (ping/cursor/runtime/focus/click/scroll/bounds/capture), so the
InputHelperService interface is unchanged.

- Win32 interop compiled once (native exe), not per call.
- PerMonitorV2 DPI via manifest so click/capture coordinates stay correct on
  mixed-DPI multi-monitor setups.
- capture returns base64 PNG bytes inline (imageBase64) instead of writing a
  temp file per frame; the client handles both base64 and the PowerShell path.
- InputHelperClient prefers the exe and falls back to the embedded PowerShell
  helper when the exe is absent, so the app still runs without the .NET build.
- main.ts resolves the exe (INPUT_HELPER_EXE env -> packaged resources/input-helper
  -> native/input-helper/bin/publish). electron-builder ships it via extraResources.
- npm run helper:build; README documents the build + fallback.

Verified end-to-end through the compiled client: sidecar spawns, runtime info
and a base64 primary-screen capture return correctly. Build stays green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

electron/services/inputHelper.ts

@@ -382,7 +382,7 @@ class InputHelperClient {
   private starting: Promise<void> | null = null;
   private disposed = false;
 
-  constructor(private readonly scriptUserDataPath: string) {}
+  constructor(private readonly options: { scriptUserDataPath: string; exePath?: string | null }) {}
 
   private async ensureStarted() {
     if (this.child) return;
@@ -396,14 +396,31 @@ class InputHelperClient {
   }
 
   private async start() {
-    const scriptPath = path.join(this.scriptUserDataPath, "input-helper.ps1");
+    // Prefer the compiled C# sidecar (ADR-008). If it is missing or fails to
+    // start, fall back to the embedded PowerShell helper so the app keeps working
+    // on machines where the native exe was never built.
+    if (this.options.exePath) {
+      try {
+        await this.startWith(spawn(this.options.exePath, [], { windowsHide: true, stdio: ["pipe", "pipe", "pipe"] }));
+        return;
+      } catch {
+        this.teardownChild();
+      }
+    }
+    await this.startWith(await this.spawnPowershell());
+  }
+
+  private async spawnPowershell() {
+    const scriptPath = path.join(this.options.scriptUserDataPath, "input-helper.ps1");
     await fs.mkdir(path.dirname(scriptPath), { recursive: true });
     await fs.writeFile(scriptPath, INPUT_HELPER_SCRIPT, "utf8");
-
-    const child = spawn("powershell.exe", ["-NoProfile", "-ExecutionPolicy", "Bypass", "-File", scriptPath], {
+    return spawn("powershell.exe", ["-NoProfile", "-ExecutionPolicy", "Bypass", "-File", scriptPath], {
       windowsHide: true,
       stdio: ["pipe", "pipe", "pipe"],
     });
+  }
+
+  private async startWith(child: ChildProcessWithoutNullStreams) {
     child.stdout.setEncoding("utf8");
     child.stdout.on("data", (chunk: string) => this.handleStdout(chunk));
     child.stderr.setEncoding("utf8");
@@ -419,10 +436,22 @@ class InputHelperClient {
     });
     this.child = child;
 
-    // First request compiles the Win32 interop; give it extra time.
+    // The C# sidecar answers ping immediately; the PowerShell fallback compiles
+    // Win32 interop on the first request, so give it extra time.
     await this.send("ping", {}, 20000);
   }
 
+  private teardownChild() {
+    const child = this.child;
+    this.child = null;
+    this.buffer = "";
+    try {
+      child?.kill();
+    } catch {
+      // Child was never spawned or already gone.
+    }
+  }
+
   private handleStdout(chunk: string) {
     this.buffer += chunk;
     let newlineIndex = this.buffer.indexOf("\n");
@@ -485,8 +514,8 @@ export interface InputHelperService {
   dispose(): void;
 }
 
-export function createInputHelperService(options: { userDataPath: string }): InputHelperService {
-  const inputHelper = new InputHelperClient(options.userDataPath);
+export function createInputHelperService(options: { userDataPath: string; exePath?: string | null }): InputHelperService {
+  const inputHelper = new InputHelperClient({ scriptUserDataPath: options.userDataPath, exePath: options.exePath ?? null });
 
   async function request(op: string, params: Record<string, unknown> = {}, timeoutMs = 8000) {
     return inputHelper.request(op, params, timeoutMs);
@@ -590,16 +619,24 @@ export function createInputHelperService(options: { userDataPath: string }): Inp
 
   async function capturePrimaryScreenViaGdi() {
     const result = await request("capture", {}, 15000);
-    const capturePath = String(result.path);
-    const buffer = await fs.readFile(capturePath);
-    await fs.unlink(capturePath).catch(() => undefined);
+    // The C# sidecar returns PNG bytes inline (no temp file). The PowerShell
+    // fallback writes a temp PNG and returns its path.
+    let base64: string;
+    if (typeof result.imageBase64 === "string" && result.imageBase64) {
+      base64 = result.imageBase64;
+    } else {
+      const capturePath = String(result.path);
+      const buffer = await fs.readFile(capturePath);
+      await fs.unlink(capturePath).catch(() => undefined);
+      base64 = buffer.toString("base64");
+    }
     const captureTargetRaw = typeof result.captureTarget === "string" ? result.captureTarget : "";
     const captureTarget: "primary-screen" | "genshin-client" = captureTargetRaw === "primary-screen" || captureTargetRaw === "genshin-client"
       ? captureTargetRaw
       : "primary-screen";
 
     return {
-      dataUrl: `data:image/png;base64,${buffer.toString("base64")}`,
+      dataUrl: `data:image/png;base64,${base64}`,
       width: Number(result.width),
       height: Number(result.height),
       originX: Number(result.originX),