feat: add CDPServer to drive lightpanda with Playwright/Puppeteer

[arrufat]

What

Adds CDPServer / AsyncCDPServer, which spawn lightpanda serve (the browser's native Chrome DevTools Protocol server) on a free localhost port and expose ws_endpoint / http_endpoint for any CDP client:

from lightpanda import CDPServer
from playwright.sync_api import sync_playwright

with CDPServer() as server, sync_playwright() as p:
    browser = p.chromium.connect_over_cdp(server.ws_endpoint)
    page = browser.new_context().new_page()
    page.goto("https://example.com")

The same endpoint works for Node Puppeteer (puppeteer.connect({ browserWSEndpoint })), Node Playwright (chromium.connectOverCDP), chromedp, etc. No Chromium is involved and the package still has zero runtime dependencies.

It is a separate process from Browser: the binary refuses mcp --port together with --cdp-port, and MCP sessions and CDP browser contexts are unrelated anyway.

How

• client.py: the spawn loop moves out of Client.__init__ into a shared _spawn(binary, mode, ...) helper. _reserve_port(port=0) serves both the free-port path (retried on a bind collision, as before) and a caller-pinned port=, which reports "port N is already in use" instead of an opaque startup failure.
• cdp.py: CDPServer (sync, context manager, __del__ cleanup, Linux PDEATHSIG via the shared spawn path) and AsyncCDPServer (lazy start() over asyncio.to_thread, mirroring AsyncBrowser's shape).
• Endpoints use the 127.0.0.1 literal on purpose: the server only upgrades on path /, rejects any Origin header, and only accepts an IP-literal or localhost Host.

Tests

• tests/test_cdp.py (stdlib only): endpoints and /json/version, pinned port, port in use, serve flag passthrough, close(), async lazy start. The PDEATHSIG test in test_browser.py is now parametrized over Browser and CDPServer.
• tests/test_cdp_playwright.py: real connect_over_cdp sessions through both the ws and http endpoints. playwright is added to the dev dependency group and the CI wheel-test venv only; connect_over_cdp needs the pip package, not a browser download, so there is no playwright install step. The tests skip when it is absent.
• Also verified by hand with Node puppeteer-core from the demo repo against a CDPServer endpoint.

Docs

README gets a "Drive it with Playwright or Puppeteer" section; the package docstring points at CDPServer. The docs site's Python reference will need a section for the new classes once this ships.