Support TiledNdArray range encoding for large coverages

When a Grid coverage's values array is impractically large to send inline, deliver the range as a CoverageJSON `TiledNdArray` (a `TileSet` with a URL template) instead of an inline `NdArray`, so clients can fetch tiles progressively.

ADR-0004 names `TiledNdArray` as the standard mechanism for tiled delivery of a large coverage -- and as the reason a separate XYZ `/tiles` route was dropped -- but no issue tracked it, so this files it (found by the roadmap-realignment audit; was Story 12).

## Scope

- `TileSet` URL-template generation for a coverage's range.
- A threshold at which the modeler emits `TiledNdArray` instead of `NdArray` (e.g., output cells above a configurable ceiling).
- Optionally, a per-tile backing resource that the URL template points at.
- Tests, including schema validation of the tiled range.

`covjson-pydantic` 0.8.0 already ships `TiledNdArrayFloat` / `TiledNdArrayInt` and `TileSet`.

## Status

Deferred (P3): no consumer needs it yet. No milestone -- a deferred remainder, filed so ADR-0004's reference is not dangling. Schedule when a concrete large-coverage need arises.

Start with ADR-0004 and locate the Grid coverage modeler that serializes the range, then inspect existing schema-validation tests and the available covjson-pydantic TiledNdArray and TileSet types. Done means a configurable size threshold selects a valid tiled range representation, with URL-template backing and tests covering schema validation.

Written by the indexing model from the issue text.