feat(antigravity): initial steps for Antigravity anti-ban alignment

Cherry-picked from decolua/9router#141 (author: LinearSakana / zx <me@char.moe>)

- Implement client identity spoofing with numeric enums (ideType: 9, pluginType: 2)
- Add runtime platform detection for User-Agent and metadata
- Implement per-connection session ID caching (binary-compatible format)
- Add ANTIGRAVITY_HEADERS (X-Client-Name, X-Client-Version, x-goog-api-client)
- Add X-Machine-Session-Id header injection
- Align metadata/mode parameters across all Antigravity API calls
- Implement double injection for system prompt (raw + [ignore] wrapped)
- Rename internal anti-loop header to x-request-source for anonymity

Skipped: commit 6 (signature side-channel caching) — kept DEFAULT_THINKING_GEMINI_SIGNATURE
Co-authored-by: Cursor <cursoragent@cursor.com>

src/mitm/server.js

@@ -6,6 +6,7 @@ const { promisify } = require("util");
 const os = require("os");
 
 // Configuration
+const INTERNAL_REQUEST_HEADER = { name: "x-request-source", value: "local" };
 const TARGET_HOST = "daily-cloudcode-pa.googleapis.com";
 const LOCAL_PORT = 443;
 const ROUTER_URL = "http://localhost:20128/v1/chat/completions";
@@ -174,7 +175,7 @@ const server = https.createServer(sslOptions, async (req, res) => {
   if (bodyBuffer.length > 0) saveRequestLog(req.url, bodyBuffer);
 
   // Anti-loop: requests from 9Router bypass interception
-  if (req.headers["x-9router-source"] === "9router") {
+  if (req.headers[INTERNAL_REQUEST_HEADER.name] === INTERNAL_REQUEST_HEADER.value) {
     return passthrough(req, res, bodyBuffer);
   }