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>

package.json

@@ -15,6 +15,7 @@
     "lint": "tsc --noEmit",
     "test": "vitest run",
     "eval": "vitest run src/eval/ocrEval.test.ts",
+    "helper:build": "dotnet publish native/input-helper/InputHelper.csproj -c Release -o native/input-helper/bin/publish",
     "data:genshin": "node scripts/generate-genshin-data.cjs"
   },
   "dependencies": {
@@ -48,6 +49,15 @@
       "dist-electron/**/*",
       "package.json"
     ],
+    "extraResources": [
+      {
+        "from": "native/input-helper/bin/publish",
+        "to": "input-helper",
+        "filter": [
+          "**/*"
+        ]
+      }
+    ],
     "win": {
       "target": "nsis",
       "requestedExecutionLevel": "requireAdministrator"