Add PERFORMANCE.md and link it plus STEALTH_PATCHES.md from README

Documents measured single-request and concurrency (5x/20x) overhead of
the context.request fast path against aiohttp: ~15-20ms/request steady
state, no serialization bottleneck through the shared context up to at
least 20 concurrent requests, browser launch (~1s) paid once at
startup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0116i7dYLNZTXNR7Lqf439bV

README.md

@@ -7,6 +7,12 @@ persistent, stealth-patched Chromium instance. Every request goes out
 through the real browser's own network stack, so it carries a genuine
 Chrome TLS/HTTP2/UA fingerprint instead of Python's OpenSSL-based one.
 
+This README is the complete documentation. Two companion documents go
+deeper on specific topics: [`STEALTH_PATCHES.md`](./STEALTH_PATCHES.md)
+(every anti-automation patch applied, and why) and
+[`PERFORMANCE.md`](./PERFORMANCE.md) (measured latency and concurrency
+overhead against `aiohttp`).
+
 ## Why this exists
 
 `requests`, `httpx`, and `aiohttp` all negotiate TLS through Python's
@@ -33,7 +39,9 @@ endpoint:
 
 Distinct hashes, produced by the same physical request path our
 `get`/`post`/etc. use — proof the fast path is not just claiming to be
-Chrome, it is Chrome's own network stack.
+Chrome, it is Chrome's own network stack. Measured latency and
+concurrency overhead of that same fast path against `aiohttp` — steady
+state, and 5×/20× concurrent — are in [`PERFORMANCE.md`](./PERFORMANCE.md).
 
 On top of that, `stealthii` removes the JS-level automation tells a
 stock headless Chromium exposes — `navigator.webdriver`, a missing