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

[PurHur]

Summary

Implement set_error_handler() for full PHP parity: VM, LLVM JIT, and AOT/self-host where applicable.

Install user error handler; include restore_error_handler in same issue.

Context

• Not in docs/capabilities.md (or JIT-deferred only).
• North Star 1 #1044 · Self-host #1056
• Audit: php script/audit-stdlib-jit.php

Acceptance criteria

• set_error_handler() in builtin tables / SelfHostBuiltinPolicy
• VM + PHPUnit (compliance PHPT or unit test)
• LLVM JIT lowering (or explicit deferral with issue link)
• docs/capabilities.md row updated (regenerate matrix)

Related

Stdlib wave 2 tracker #1237 (closed; prior items largely landed).