planner: generalise the data layers for one layer at a time

[timohueser]

Summary

This change prepares the website planner for the Weather and Wind layers (#2329). Snow is the only layer that it implements. A later layer is one registry line, one implementation file and its tests.

• Registry. layers/registry.ts lists the data layers in menu order. Each entry names its archive and a factory. A layer shows only when the region has its archive URL. DATA_URLS in map-data.ts maps each archive name (snow, climate) to its URL. The names agree with planner: bake the climate data layer from ERA5-Land #2345 (VITE_PLANNER_CLIMATE_URL).
• One layer at a time. The layer menu has a segmented control: Off, then each layer. The map syncs the hidden layers first and then the shown layer. The layer date stays shared.
• No stats block. The left-panel stats block is gone for all layers. The one-line label above the year slider stays.
• Interface. DataLayer now has:
  • strip?: an optional profile strip (legend, fills, values).
  • variable?: an optional switch in the date bar (for example Temperature | Rain). The layer owns its value.
  • year(samples | null): the year slider as a Grid with labelled rows, also empty rows before a route exists.
  • chart(): a Chart with labelled grids, an optional extra component (for example a wind rose) and an optional note.
  • legend(): swatches or a continuous scale (LayerLegend.svelte, solid colours).
  • sample(line): gets the line coordinates and the profile heights, so a layer can compute the bearing and correct temperature without new requests.
  • Grids have 183 two-day columns or 52 weeks.
• Snow. The snow layer keeps its look. The tree note and the forest query moved from PlannerMap into the snow layer. LayerSpot merged into LayerInspect (compact).

Not in this change: the night-low labels at overnight stops (Weather). They need the trip day ends as an input. A later change can add them.

Checks

• npx vitest run src/lib/planner src/components/planner: 163 passed
• npx svelte-check --fail-on-warnings: no new errors. The 15 that remain are missing wasm pkg/ build outputs.
• npm run test:planner: the request budget passes, with no change
• obc suites check, obc prose --check: OK
• Manual check on the Engadin data (light and dark): menu, map, date bar, strip popover, map callout

Public docs changed: no

Requirements: none

🤖 Generated with Claude Code

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