PointSeries domain (deferred: needs a temporal resolver backing)

**Deferred: PointSeries domain (needs a temporal resolver backing).**

> **Re-scoped for the slicewise direction ([ADR-0001](docs/adr/0001-covjson-http-api-direction.md)).** This issue originally bundled Point, PointSeries, and CoverageCollection as a wide "modeler part 2." Under the vertical-slice approach those no longer travel together:
> - **Point** ships end-to-end in the `/position` slice (#44).
> - **CoverageCollection** is tracked separately (#24).
> - **PointSeries** remains here, and is what this issue now tracks.

PointSeries needs a real `t` axis, which requires the temporal resolver backing that ADR-0001 explicitly defers (a time-dimensioned NetCDF/Zarr via xarray, or a STAC-backed seam). It is therefore blocked until that backing lands, at which point it becomes its own vertical slice (`/cube` and/or a temporal `/position`) that adds `PointSeriesInput` + the `t`-indexed range path on top of the Point work from #44.

Scope when unblocked:

- `input.py`: `PointSeriesInput(_CoverageInputBase)` (`geometry: Point`, `timestamps`; shape contract `data.shape[1] == len(timestamps)`), extending the `CoverageInput` union.
- `modeler.py`: a `case PointSeriesInput()` arm adding a `t` `ValuesAxis` from `timestamps` and `numpy_dtype_to_ndarray(data[i], dtype, ("t",))` -> `shape=[n_times]`, `axisNames=["t"]`; temporal referencing via `create_temporal_reference`.
- Tests: `t`-indexed range, temporal referencing, masked -> `null`, schema-validated.