Stdlib: DateTime::diff() / DateTimeImmutable::diff() VM methods (#3162)

[PurHur]

Summary

• Register DateTime::diff() and DateTimeImmutable::diff() on the VM, delegating to existing DateTimeSupport::diffDateTimes() (php-src php_date_diff / zim_DateTime_diff).
• Add compliance PHPT and maintainer repro; day counts match Zend PHP 8.2.

php-src reference

• ext/date/php_date.c — PHP_METHOD(DateTime, diff), php_date_diff()

Verification

docker info >/dev/null
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/parity_datetime_diff.php
php bin/vm.php test/repro-maintainer/parity_datetime_diff.php
vendor/bin/phpunit --filter datetime_diff
php bin/compile.php -l test/repro-maintainer/parity_datetime_diff.php
'

Output (VM repro):

7
forward
7

PHPUnit: OK (2 tests, 2 assertions)

AOT compile-only (-l) passes. Full native AOT execution of DateTime::diff() remains blocked on DateTime method JIT lowering (#4043), same as other DateTime OOP methods.

Closes #3162 (partial — OOP diff(); DatePeriod / full JIT/AOT tracked separately).

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