Support running on stock PHP (without TrueAsync fork)

[EdmondDantes]

Today the extension requires the TrueAsync php-src fork because it depends on the native coroutine and udp_bind APIs from ext-async. Investigate and implement a path to run on stock upstream PHP 8.6+.

Goals

• Detect at build time whether the host PHP has the TrueAsync async API; fall back to a non-async I/O strategy when it doesn't.
• Provide a synchronous / fiber-based execution model on stock PHP — request handlers run on PHP Fibers scheduled on an internal libuv loop owned by the extension.
• Preserve the public HttpServer API surface so user code does not have to change.
• Document the feature/perf delta clearly: no shared-loop integration with user code, no udp_bind (so HTTP/3 may be limited or unavailable on stock PHP), no deep coroutine integration with user-space libraries.

Open questions

• Can HTTP/3 work on stock PHP if the extension owns the UDP socket itself, bypassing the missing udp_bind?
• How is the event loop driven from a stock PHP CLI script — a blocking $server->start() that owns the thread, vs. an explicit $server->tick()?
• What's the minimum PHP version we can realistically target (8.1 for Fibers, 8.6 for current TrueAsync features)?

Acceptance

• pecl install (or equivalent) on a vanilla PHP 8.x build produces a working extension.
• Hello, World! quickstart from the README runs unchanged on stock PHP.
• Docs clearly state which features require the TrueAsync fork.