CVE-2026-7260: fix phar circular symlink crash by replacing recursive
phar_get_link_source() with an iterative Floyd cycle-detection walk in
ext/phar/util.c (GHSA-vc5h-9ppw-p5f3); add regression test
ext/phar/tests/tar/ghsa-vc5h-9ppw-p5f3-symlink-circular.phpt