AOT: string assignment uses addref instead of eager memcpy (#36192)

[PurHur]

Summary

• Value-box string copies ($u = $s, boxing native strings, coerceToValuePtrForStore) now call __ref__addref instead of __string__separate (Zend zend_string_copy semantics).
• __string__separate remains for mutation / hashtable-key ownership paths.
• Adds AOT fixture + unit lint test + perf repro script.

Verify

./script/phpunit.sh --filter 'StringCopyOnWriteAotTest|aot_string_copy_addref_36192'
# OK (2 tests, 8 assertions)

./script/aot-smoke.sh
# aot-smoke: 9 passed, 0 failed

make dev-verify-fast
# dev-verify-fast: OK (99s wall)

make north-star5-verify-fast
# north-star5-verify: OK (fast)

# Perf repro (1 MB string × 20k aliasing assigns):
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/compile.php -o /tmp/s test/repro/string_copy_addref_36192.php && time /tmp/s'
# AOT: 0.004s (was 0.754s on master); Zend: 0.010s; output 20971520000

Closes #36192

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