fix(solver-problems): compare RULE_LEARNED sort keys like PHP's <=>

Problem::getPrettyString sorts same-priority reasons by
getSortableString(), whose RULE_LEARNED key is a '-'-joined literal id
string (e.g. "-95"). PHP's <=> compares two numeric strings
numerically, but the port used plain String::cmp (byte-wise), which
reverses relative order for same-length negative-number keys. Added
shirabe_php_shim::loosely_compare to approximate PHP's <=> for this
pattern (numeric compare when both sides parse as numbers, else byte
compare) and switched the sort comparator to use it.

The diagnosis this replaces (from the commit being amended) blamed
Pool package-id assignment order diverging from PHP under
COMPOSER_POOL_OPTIMIZER=0. That was disproven this session: direct
instrumentation of both PHP and the Rust port confirmed identical
relative package-id order, including on the ~335-package
github-issues-7665 fixture (ids matched up to a constant +2 offset
from a platform-mock package count difference). The sort comparator
was the actual bug, not package loading order.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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