PR #60 merged: fix: pin composer platform to php 8.3 so the lock installs on minimum PHP

Follow-up to #57 — my lock refresh was resolved on this sandbox's PHP 8.4, which pulled Symfony 8.1 (php >=8.4.1). The lock audited clean but cannot install on PHP 8.3, which is what the Security Check workflow (and any 8.3 contributor) runs — so the nightly went from 19 advisories to incompatible lock.

Root-cause fix rather than a re-roll: config.platform.php = 8.3.0 makes composer always resolve as the package's minimum PHP, so the lock can never drift above what supported environments can install. Resolved lock lands on Symfony 7.4.14 LTS (advisories fixed in 7.4.12) — composer audit --locked: 0 advisories.

Consumers are unaffected either way (library locks aren't used by consumers) — v1.6.1 needs no re-release; this is repo-CI hygiene.

Full pest suite green (63 passed).