v0.4.0 — Other / not in the list escape hatch

A first-class "Other / not in the list" escape hatch for make/model pickers, so a vehicle the dataset does not cover is never a dead end:

• include_other: on makes, models, make_options, and model_options appends the option (name lists get the label; option pairs get the stable "other" slug). Idempotent.
• allow_other: true on the vehicle_make / vehicle_model validators accepts it, so the dropdown and the validation agree by construction.
• config.other_label (default "Other") localizes the label; Vehicles.other? recognizes the label or the canonical "other" slug (case/diacritics-forgiving), and Vehicles.other_label reads it back.

Opt-in and backwards compatible — existing callers are untouched.

Full Changelog: v0.3.0...v0.4.0