fix: add rwlock for implicit opcache restarts (OOM/hash overflow)

[superdav42]

Summary

Adds a pthread_rwlock around the request lifecycle to protect against implicit opcache restarts (OOM, hash overflow) that bypass the opcache_reset() override in #2073.

Problem

PR #2073 intercepts explicit opcache_reset() calls and drains all threads before the reset. However, WordPress (and other apps) trigger implicit opcache restarts through a different path:

1. opcache_invalidate() fills opcache memory over time
2. opcache detects OOM → calls zend_accel_schedule_restart(ACCEL_RESTART_OOM)
3. Sets restart_pending = true
4. Next php_request_startup() → accel_activate() sees restart_pending → accel_is_inactive() → proceeds with accel_interned_strings_restore_state() (memset on shared memory)
5. Other threads are mid-request reading from that shared memory → crash

This path never calls opcache_reset(), so #2073's override doesn't fire.

Production evidence

Configuration	Crash frequency
No fix	Every 20-60 min
#2073 alone	Every 10-12 min (no improvement)
rwlock alone (our #2349)	Every 6-12 hours (partial — missed worker threads)
Combined (this PR)	Testing in progress

Fix

Uses PHP's zend_accel_schedule_restart_hook (PHP 8.4+, declared in Zend/zend.h) to detect when an implicit restart is scheduled, then serializes the next php_request_startup() via a pthread_rwlock:

• Normal requests: read lock (concurrent, single atomic CAS — zero measurable overhead)
• When hook fires: sets an atomic flag
• Next php_request_startup(): sees flag → acquires write lock (exclusive), blocks until all current requests finish php_request_shutdown() and release their read locks
• Inside exclusive startup: accel_is_inactive() correctly sees no active threads, reset proceeds safely
• After startup: write lock released, all threads resume

Why both fixes are needed

• fix: safe opcache_reset #2073: Covers explicit opcache_reset() calls — drains threads via Go state machine, works on all PHP versions
• This PR: Covers implicit OOM/hash-overflow restarts — serializes via rwlock, PHP 8.4+ only (hook doesn't exist before)

The rwlock is unconditional (all PHP versions get the lock around requests) but the hook registration is #if PHP_VERSION_ID >= 80400. On PHP 8.2/8.3, the rwlock adds negligible overhead with no functional benefit for implicit restarts.

Changes

• frankenphp.c: +50 lines
  • pthread_rwlock_t frankenphp_opcache_rwlock — read-write lock
  • frankenphp_opcache_restart_hook() — sets atomic flag (PHP 8.4+)
  • php_thread() loop: read lock before php_request_startup(), unlock after php_request_shutdown(), write lock when restart flag is set
  • Hook registration after frankenphp_sapi_module.startup()