fix(tuf): preserve rollback floors and recheck target authorization

[wolfv]

Stack: follows #185.

• Preserve authenticated timestamp/snapshot version floors even after cached metadata expires.
• Recheck root, timestamp, snapshot, targets expiry and version pins at target resolution, including after unsuccessful refreshes.
• Bound HTTP retrieval with an overall deadline in addition to inactivity timeouts.
• Add regressions for cross-process replay and target access after expiry/failed refresh.

Validation: cargo test and clippy for sigstore-tuf and sigstore-trust-root, all features.

Signed-off-by: Wolf Vollprecht w.vollprecht@gmail.com