Language: magic __get/__set pre/post dec no Undefined property (#32014)

[PurHur]

Summary

• Add repro scripts and VM/JIT compliance .phpt for --$obj->n / $obj->n-- with __get/__set — no Undefined property warning, values match Zend (dec=0, postdec=0).
• Behavior is already correct via the Language: $obj->n++ with __get/__set warns Undefined property — Zend silent magic (zend_object_handlers.c) #31992 guard in lib/VM.php / lib/JIT.php (propertyFetchDestUsedAsIncDec includes PRE_DEC/POST_DEC; skip warn when propertyReadUsesMagicGet).
• Register Language: $obj->n++ with __get/__set warns Undefined property — Zend silent magic (zend_object_handlers.c) #31992 inc and Language: $obj->n-- / --$obj->n with __get/__set warns Undefined property — Zend silent magic (re-#31992, zend_object_handlers.c) #32014 dec compliance suites in phpunit.xml.dist.

php-src reference

• Zend/zend_object_handlers.c — magic read/write for inc/dec
• Zend/zend_vm_def.h — ZEND_PRE_DEC_OBJ, ZEND_POST_DEC_OBJ

Verification

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_magic_prop_dec.php'
# dec=0

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_magic_prop_postdec.php'
# postdec=0

./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_magic_prop_dec.php'
# dec=0

./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_magic_prop_postdec.php'
# postdec=0

./script/phpunit.sh --filter 'MagicPropDecNoUndefWarn|MagicPropIncNoUndefWarn'
# OK (4 tests, 4 assertions)

Not covered: AOT magic property inc/dec value correctness (pre-existing gap — AOT returns stale inc=1/dec=1; tracked separately in AOT inventory).

Closes #32014

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