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

[PurHur]

Summary

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

Return stack trace array; debugging and error handlers.

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

• debug_backtrace() 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).

[PurHur]

Implementation PR #1404 is open but needs rebase onto current master (merge conflicts). Docs tracker: docs/roadmap-wave3.md — last open item in wave-3 stdlib batch.

[PurHur]

Still open — #1404 needs rebase. Last stdlib item in wave-3 batch (12/13 closed otherwise).

[PurHur]

JIT lowering scaffold in JitDebugBacktrace.php (PR incoming); VM-only until MCJIT frame-array returns are stable.

[PurHur]

Follow-up from PR #1854: `JitDebugBacktrace` lowering is in the M2 spine bundle and uses `HashTableHelper::packVariables`, but enabling `debug_backtrace::call()` still MCJIT-segfaults (exit 139) in Docker JIT tests. VM path unchanged. Track under #1751.