Fix DECLARE_TRAIT MCJIT segfault — defer to VM lowering (#6284)

[PurHur]

Summary

• Add Block::containsDeclareTraitOpcodesInScriptScope() and wire it into requiresVmLowering() so any compilation unit containing TYPE_DECLARE_TRAIT skips MCJIT link/execute (exit 139) until native lowering is stable (Language: __TRAIT__ scope semantics — empty outside trait body (Zend zend_compile.c parity) #3609).
• Adds maintainer repro test/repro/trait_empty_mcjit_probe.php and TraitEmptyMcjitCompileTest (@group llvm).

This extends the existing trait-__construct deferral gate to all trait declarations, fixing bin/jit.php -l on empty traits without touching runtime/*.c.

Remaining (follow-up for native MCJIT)

• Trait constructor promotion still VM-fallbacks via containsTraitConstructorOpcodes() (redundant but harmless).
• TraitConstructorJitExecuteTest stays skipped until lib/JIT/ DECLARE_TRAIT MCJIT link is fixed.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/jit.php -l test/repro/trait_empty_mcjit_probe.php; echo exit:$?
php bin/jit.php test/repro/trait_empty_mcjit_probe.php
vendor/bin/phpunit test/unit/TraitEmptyMcjitCompileTest.php --group llvm
'

Output:

exit:0
ok
OK (3 tests, 11 assertions)

Closes #6284

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