Add convenience helpers for common joins and lookups

[dlebauer]

Follow-up from review of #12.

Goal

Make common offline workflows easier without requiring users to reconstruct the same joins manually each time.

Track

• a small API for frequent lookups and joins over packaged tables
• helpers centered on traitsview, species, variables, pfts, and management-related tables
• examples and tests for the main user workflows

Constraints

• Keep the API small, composable, and data-first.
• Do not recreate the legacy application API.

Acceptance criteria

• A small initial helper set is implemented and documented.
• Each helper has tests.
• Vignettes or examples show the intended workflows.

[dlebauer]

low priority, closing as official backlog