feat(jit): closure use (&$var) by-reference lowering (#72)

[PurHur]

Summary

• Implement JIT lowering for use (&$var) closure captures, mirroring the VM bindClosureCaptures / bindClosureCallCaptures indirect path.
• ClosureHelper::referenceCapture passes live enclosing variables through ClosureWithCaptures; compileBlock aliases by-ref capture slots to the trailing __value__* param (foreach by-ref pattern) instead of snapshotting.
• Add closure_use_byref.phpt compliance case and extend ClosureJitCompileTest IR verify coverage.

Test plan

• phpunit test/compliance/ClosureVMTest.php (includes closure_use_byref.phpt)
• phpunit test/unit/ClosureJitCompileTest.php --filter testClosureModuleVerify
• bin/jit.php -r MCJIT runtime (skipped here: probe segfaults in CI env; IR verify passes)

Limitations

• By-ref captures alias simple enclosing locals/globals resolved at closure creation; complex lvalues (array elements, properties) are not covered yet.
• MCJIT execute path for closures remains environment-sensitive (same as existing closure JIT); compile-time IR verification is the gate in ClosureJitCompileTest.
• Bootstrap / self-host AOT still stubs closures to null.

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