arch-12 §G: Add climate time-series (SPI) via REST backend

[koito19960406]

Parent: #133
Depends on: #139 (REST backend implementation)

Goal

Add the SPI (Standardised Precipitation Index) monthly time-series from Space2Stats as a wb_climate_rest REST-backend source in inst/hex_vars_registry.yaml.

This is a YAML-only PR — the REST adapter from #139 handles all the fetch logic.

YAML entry

sources:
  wb_climate_rest:
    backend: "rest"
    api_root: "https://space2stats.ds.io"
    collection: "climate"
    pop_var: ~
    variables:
      spi_3month:
        source_col: "spi_3"
        var_name: "SPI-3 Drought Index ({year})"
        var_description: |
          3-month Standardised Precipitation Index. Negative values indicate
          drought conditions; positive values indicate wet conditions.
        var_units: "dimensionless"
        pillar_group: ~
        pillar_name: "Climate hazards"
        legend_revert_colours: true
        time_col: "month"
        available_years: ~
        weight: "area"
        fun: "mean"
        fltr_exclude_pti: false

(Exact source_col and time_col values to be confirmed from #134 / the /fields endpoint.)

Verification

• get_available_years("spi_3month") returns the live month/year list from https://space2stats.ds.io/fields.
• A manual fetch for 3–5 Rwanda hex IDs returns columns spi_3month_<period>.
• list_hex_vars() shows spi_3month under the wb_climate_rest source.
• Existing testthat suite stays green.
• R CMD check 0/0/0.