JIT: Variable variables ($$name) in LLVM lowering

[PurHur]

Summary

VM done (#1226); add JIT path for dynamic local names where provably constant.

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]

JIT lowering for variable variables ($$name) merged in #1381. Capabilities and wave-3 tracker updated in docs/roadmap-wave3.md.