Explore Region Module

A region-scoped explore rail ("Keep exploring in Japan"). label is the ladder label the client already renders (drill headers, trip-anchor labels) — keys are label-built on both bake and serve, so the vocabulary matches by construction. country scopes region-rung labels. trip_id stamps trip-rail serves for per-trip funnel attribution, and dates and places the rail: with it the city rung can say how far each card is from where the trip sleeps (before it starts) or from user_lat/user_lng (during it — the client's own fix, absent without permission). today is the device-local date, clamped server-side, so the before/during call matches the user's midnight. Empty items (never 404) when the region has no pool — the client hides the rail.