Language: foreach over Iterator / IteratorAggregate objects (#3234)

[PurHur]

Summary

• VM TYPE_ITER_* opcodes now drive user Iterator / IteratorAggregate objects via rewind() / valid() / current() / key() / next() (Zend zend_iterators.c parity).
• IteratorAggregate::getIterator() is resolved on reset; non-iterable objects raise TypeError instead of LogicException.
• Adds compliance PHPT fixtures and documents the construct in capabilities-syntax.md (VM-only; JIT deferred).

php-src reference

• Zend/zend_iterators.c — iterator helpers
• Zend/zend_execute.c — ZEND_FE_RESET_R, ZEND_FE_FETCH_R

Test plan

• ./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter foreach_iterator'
• Issue repro: Iterator + IteratorAggregate print expected lines on VM
• Non-iterable object exits 255 with TypeError
• JIT lowering for object foreach (phase 2 — VM fallback acceptable v1)

Closes #3234

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