Stdlib: session_name() — VM + LLVM JIT/AOT

[PurHur]

Summary

Implement session_name() for the PHP compiler: Zend/VM path, LLVM JIT lowering, and AOT/self-host emission where applicable.

Context

• Not listed in docs/capabilities.md builtin matrix (or marked absent).
• Tracked under North Star stdlib coverage (#78, #1056).
• Run php script/audit-stdlib-jit.php after implementation.

Acceptance criteria

• session_name() registered in builtin tables / SelfHostBuiltinPolicy as appropriate
• VM implementation with PHPUnit coverage
• LLVM JIT lowering (or documented deferral with issue link)
• AOT/self-host path if used by bootstrap fixtures
• Row added/updated in docs/capabilities.md

Notes

Get/set session name.

[PurHur]

Merged in #1322: native LLVM/JIT session_name() lowering (__phpc_session_name_apply), AOT runtime storage, PHPT coverage, and M2 bundle growth. Stdlib JIT audit missing count is now 0.