Add optional runtime FST loader with safe fallback

[shuvakant6623]

This PR adds an optional runtime FST loading path so users can use newer
PURL data without upgrading the purl-validator library.

Key points:

• Default behavior is unchanged: bundled FST is used by default.
• Runtime mode is opt-in via PURL_VALIDATOR_FETCH_LATEST environment variable.
• If a runtime FST is available on disk, it is used.
• On any failure, the code safely falls back to the bundled FST.
• Offline-first and air-gapped use cases remain fully supported.

This introduces a small loader abstraction in lib.rs and a new runtime
module to handle optional runtime FST loading.

No behavior change occurs unless runtime mode is explicitly enabled.