fix: stop bundling libuv into the extension (#106)

[CodeLieutenant]

Fixes #106.

The bug

Every CMake preset set LINK_LIBUV_STATIC=ON, so cassandra.so carries its own copy of libuv. When that module is linked against a shared libcassandra.so.2 / libscylla-cpp-driver.so, ELF's flat namespace makes the driver's uv_* references bind to the copy inside cassandra.so rather than the libuv.so.1 it was compiled against. The driver then runs a libuv build that doesn't match its headers, which corrupts memory around its event loop.

In #106 that showed up as a garbage refcount and an abort inside the driver:

php: ref_counted.hpp:43: ... RetryPolicy::dec_ref(): Assertion `new_ref_count >= 1' failed.
#30 uv__finish_close ... at out/ReleaseLibCassandra/_deps/libuv-src/src/unix/core.c:351
#31 uv__run_closing_handles ...
#32 uv_run ...
#33 datastax::internal::core::EventLoop::handle_run()

Frames #30–32 are the php-driver's libuv running inside libcassandra's event-loop thread — the interposition, right there in the reporter's own backtrace.

Two things this explains:

• v1.3.8 works, v1.3.9+ doesn't. 1.3.8 built via config.m4 and linked libuv dynamically (PHP_ADD_LIBRARY(uv,...)); the CMake build introduced in 1.3.9 links it statically.
• Not reproducible on macOS. Mach-O two-level namespaces bind each dylib to its own dependencies at link time, so no interposition happens.

Reduced repro of the mechanism (gcc:13, uv_version standing in for libuv):

--- module statically links its own copy (current preset behaviour) ---
libcassandra sees uv_version() = 148     # wrong copy wins
--- same, plus -Wl,--exclude-libs,ALL ---
libcassandra sees uv_version() = 144     # correct

The fix

The extension used libuv for exactly one thing — an rwlock guarding the cassandra.log INI value — which does not justify the dependency.

• src/php_scylladb.c: uv_rwlock_t → pthread_rwlock_t, statically initialised. That also removes an ordering hazard: the log callback can fire from driver threads before php_scylladb_log_initialize() ran uv_rwlock_init(). Cleanup now takes the write lock instead of destroying the lock.
• libuv dropped from the extension build: find_package(Libuv), Libuv::Libuv, cmake/FindLibuv.cmake, the LINK_LIBUV_STATIC / BUILD_LIBUV_FROM_SRC options, the --enable-libuv-static config.m4/PIE flag, and the preset entries. libuv remains a dependency of the C/C++ driver, which is where it belongs — scripts/compile-libuv.sh and the CI action stay.
• -Wl,--exclude-libs,ALL on non-Apple platforms, so no future statically linked third-party archive (OpenSSL, GMP, a static cpp-driver) can interpose on the driver's dependencies. -fvisibility=hidden was already set, but it only covers our own objects, never archives compiled elsewhere.

Reviewer notes

• Rebased on trunk after Prepare for PHP 8.6: drop removed internal APIs, add 8.6 build presets #141 and ci: fail the build when cassandra.so has undeclared native dependencies #142. The only remaining CMakePresets.json change is the removal of the LINK_LIBUV_STATIC entries — Prepare for PHP 8.6: drop removed internal APIs, add 8.6 build presets #141 already fixed the stale PHP_DRIVER_* → PHP_SCYLLADB_* variable names.
• config.m4 had the same drift: --enable-driver-static was passing the dead -DPHP_DRIVER_STATIC and therefore doing nothing. Now -DPHP_SCYLLADB_STATIC.
• CI is unaffected by the removed option — .github/actions/build-extension never passed LINK_LIBUV_STATIC, and it links the C/C++ driver statically, so libuv still arrives through the driver's own .pc.
• Verified on macOS/arm64 (PHP 8.5, ScyllaDB cpp-driver 2.16.2): configure + build clean, the undefined symbol: cass_session_new when installing with libcassandra #106 repro script runs clean, Pest suite 860 passed. The one failure is the known local-only UuidTest cross-process case that needs the extension installed under its bare name.
• Worth double-checking on a Linux runner that nm -D --defined-only cassandra.so | grep uv_ comes back empty for a shared-driver build.

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