Language: closure use (&$var) JIT by-ref for native lvalues (#72)

[PurHur]

Summary

• Fix JIT lowering for use (&$var) when the enclosed variable is a native scalar ($x = 1): promote to a shared __value__ box at capture time and pass __value__* through ClosureWithCaptures.
• Route assignments to aliased capture slots via JitValueBox::assignToPointer (native literals and boxed values).
• VM parity was already green; add closure_use_byref_mutate.phpt for the issue repro ($f(); echo $x → 2).

php-src reference

• Zend/zend_closures.c — zend_create_closure, bound variable table
• Zend/zend_compile.c — zend_compile_var_dep / use (&$x)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/ClosureVMTest.php --filter "closure_use_byref"'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/unit/ClosureJitCompileTest.php --filter testClosureModuleVerify'
php bin/vm.php -r '$x = 1; $f = function () use (&$x) { $x = 2; }; $f(); echo $x;'
# → 2

Note: bin/jit.php MCJIT execute still segfaults in this harness (exit 139; pre-existing #98). LLVM module verify passes for all closure fixtures including closure_use_byref_mutate.phpt. AOT closure link remains out of scope (#1492).

Closes #72

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