Language: Array to string warning on string-offset assign (#22925)

[PurHur]

Summary

• String-offset assign of an array now emits Zend's Array to string conversion warning before Only the first byte will be assigned…, then writes 'A' (first byte of "Array").
• VM: Variable::byteFromAssignValue — TYPE_ARRAY path + emitArrayToStringWarning fallback Context from the offset slot.
• JIT: StringOffsetRuntime::assignByte handles TYPE_HASHTABLE with the same two warnings.
• Compliance + repro guards.

Closes #22925

php-src reference

• Zend/zend_execute.c — zend_assign_to_string_offset
• Zend/zend_operators.c — array → string conversion

PHP implementation

• lib/VM/Variable.php (php-in-PHP; no new C)
• lib/JIT/Builtin/StringOffsetRuntime.php + StringOffsetJitHelper::ARRAY_TO_STRING_WARNING

Trust probe (before claim)

Gate	Result
bootstrap-inventory --check	ok OK 6518/6518 (post-fix)
north-star5-fast	red — gen-0 manifest/driver mismatch (#8713 / #22642), pre-existing
vm-driver-probe	ok
user_release_ready	no (same north-star5-fast)

Test plan

• Issue repro:

php bin/vm.php test/repro/issue_22925_string_offset_array_assign_warning.php
# W:Array to string conversion
# W:Only the first byte will be assigned to the string offset
# 'Ab'

• vendor/bin/phpunit --filter string_offset_array_assign_warning → OK
• Related string_offset_* VM compliance → OK (17)
• php script/bootstrap-inventory.php --check → OK 6518/6518

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