Language: WeakReference and WeakMap

[PurHur]

Summary

PHP 8 weak references; needed for some runtime patterns and future GC semantics.

Context

• Roadmap: #78, North Star 2 #1056
• Syntax matrix: docs/capabilities-syntax.md · Lint gaps: docs/unsupported-syntax.md

Acceptance criteria

• Parser + CFG lowering (php-cfg / Compiler)
• VM semantics + compliance or unit tests
• LLVM JIT lowering (or documented deferral in SelfHostBuiltinPolicy)
• AOT/self-host probe if on vm.php / compile.php path
• Row in docs/capabilities-syntax.md (regenerate via script/capability-syntax.php)

Related

See open language backlog #1238.

[PurHur]

Open PR #1478 (VM stub). Not merged yet — tracked in docs/roadmap-wave3.md.

[PurHur]

Merged #1478 on master.