test(aot): add bin2hex PHPT for LLVM compile-and-run

[PurHur]

Summary

• Add test/fixtures/aot/cases/bin2hex.phpt exercising bin2hex() through bin/compile.php (empty string, binary bytes, ASCII).
• Update docs/capabilities.md to record AOT PHPT coverage for bin2hex.

Test plan (Docker, local harness)

Image: php-compiler:22.04-dev, LLVM at /opt/llvm9 (preinstalled in dev image).

tar -cf - --exclude='.git' --exclude='.llvm' . | docker run --rm -i -e PHP_COMPILER_LLVM_PATH=/opt/llvm9 -w /compiler php-compiler:22.04-dev bash -c '
  tar -xf - && script/apply-patches.sh >/dev/null 2>&1
  php script/capability-matrix.php --check
  php vendor/bin/phpunit test/aot/AotTest.php
'

Results:

• script/capability-matrix.php --check — OK (105 builtins)
• test/aot/AotTest.php — OK (43 tests, 50 assertions) including new bin2hex case

Baseline survey: 42 AOT PHPT fixtures + PHPUnit web/refresh tests; LLVM present in container (not skipped).

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