perf(optimizer): inline count for known array variables

[yavon007]

Single-argument count($items) still calls the generic php::fn::count when $items already has a fixed array representation. The runtime checks the type before returning the array's stored element count.

Lower that case to static_cast<php::Int>(items.count()). The signed cast preserves PHP integer arithmetic. The shortcut accepts only known local array variables and retains argument-safety checks; recursive/two-argument calls, global wrappers, dynamic values, objects and compound expressions keep their existing paths.

Adds codegen and runtime regression coverage plus a standalone benchmark with raw results and reproduction instructions.

Performance

Linux ARM64 in Docker, PHP 8.5.10 ZTS, PHPX 6a68f38, GCC -O2, no LTO; baseline 692841a6. A 30-million-iteration array-update/count workload decreased from a median 319.63 ms to 192.73 ms (39.70% less elapsed time). All nine alternating measured pairs were faster after a discarded warm-up pair; checksums matched native PHP. Array updates keep the count from being a loop-invariant-only measurement.

These are isolated workload measurements on a shared development machine, not an application-wide improvement or a comparison of execution speed against native PHP.

Validation

• CountLiteralFoldTest|CountGlobalsTest: 6 tests / 10 assertions pass; the added codegen test fails on the original implementation.
• Four targeted PHPTs pass: known-array mutation/recursive/COW counting, Countable dispatch, invalid-type errors, and global-variable access. Codegen coverage also checks that $GLOBALS and $_SERVER retain runtime calls.
• Native PHP and compiled benchmark checksums match (2999995050).
• git diff --check passes. Full-suite and cross-platform validation were not run.