PBDB interval fallback
- paleobiology_list_intervals falls back to a live PBDB lookup for sub-stage and regional interval names (e.g. Late Maastrichtian, Lancian) the bundled ICS snapshot does not carry, labeled with a new source field and the resolving time scale (#19)
- new interval_lookup_unavailable error (retryable) keeps an unreachable PBDB from reading as a nonexistent interval; level widens to accept any PBDB rank string
- fixed pre-existing docs/design.md drift (stale snapshot filename, two removed IntervalIndex methods, wrong interval count) and a false no-network-call claim in README/CLAUDE.md/AGENTS.md