Language: keep arrow captures beside null in new args (#31720)

[PurHur]

Summary

• fn() => new Pair($x, null) / new Pair(null, $x) dropped auto-captured $x on VM (both args became null) while Zend kept the capture.
• Root cause: positional hoisted null/true/false ConstFetch folding treated named CV / Phi temps as dead inline args.
• Fix: exclude named locals from that positional map; force compileOperand for named CVs when a trailing bool/null ConstFetch prelude is present; do not treat named CVs as enum prelude slots.

Closes #31720

php-src: Zend/zend_compile.c (arrow auto-capture + ZEND_AST_NEW)

Test plan

• ./script/docker-exec.sh -- bash -lc 'php test/repro/maintainer_gap_arrow_new_null_arg.php' (Zend) matches VM
• ./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_arrow_new_null_arg.php'

  mixed_null a='captured' b=NULL
  null_then_cap a=NULL b='captured'
  int_control a='captured' b=0
  dup_control a='captured' b='captured'
  long_use a='captured' b=NULL
  

• ./script/phpunit.sh --filter ArrowNewNullCapture31720Test → OK (2 tests, 11 assertions)

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