ILAMB: mrsos-WangMao and emp/GLEAM still blocked

## Status

Follow-up from the ILAMB obs4REF migration.

### `lai-AVH15C1` -> obs4MIPs (DONE)

Migrated to the obs4MIPs `NOAA-NCEI-LAI-AVHRR-5-0` reference (ESGF) and re-minted. Solve migration is green (intake-esgf fetch into the fixture, dict-form yaml, `ilamb.txt` prune, regenerated snapshot). Native baselines re-minted on gus (cmip6 + cmip7), applied at `diagnostic_version: 3`; full ILAMB unit suite passes.

Notable: the reference is ~0.05 deg (5 km, 4.8 GB), far finer than any CMIP model, which hung ilamb3. Added a reusable `climate_ref_coarsen_spatial` transform (conservatively coarsens the reference to 0.5 deg before comparison).

### `mrsos-WangMao` -> blocked: file is not obs4MIPs-compliant

The CEDA `obs_for_ref_v2` WangMao file (`ORNL/WangMao/mon/mrsos/50km/v20250321/...`, 9.3 GB) carries no obs4MIPs global attributes -- no `activity_id`, `source_id`, `grid_label`, or `nominal_resolution`; only `contact`, `created on`, and `note = "OLC ORS"`. `parse_obs4mips` rejects it, so it cannot be ingested or produce a solve-fixture row. Distinct from the FLUXNET naming split (#784): there the metadata is inconsistent, here it is absent. Blocked until the file is re-formatted into obs4MIPs upstream.

### `emp-GLEAMGPCP2.3` -> blocked: no obs4REF equivalent

No obs4REF equivalent for GLEAM evapotranspiration (`et`/`evspsbl`) yet. The `ilamb/evspsbl/GLEAMv3.3a/et.nc` registry entry and string-form source remain until one is published.

`pr/GPCPv2.3` and `tas/CRU4.02` stay in `ilamb.txt` regardless -- they back `gpp-WECANN` relationship analyses.

### Tooling added

`scripts/fetch_obs4mips_fixture.py` (`make fetch-obs-fixture`) uses intake-esgf to grab obs4MIPs reference data from ESGF and merge full-fidelity rows into the solve fixture. `climate_ref_coarsen_spatial` transform for very-fine references.

Start by reading scripts/fetch_obs4mips_fixture.py and the obs4MIPs parsing path around parse_obs4mips. The WangMao file needs upstream obs4MIPs formatting before it can produce a solve-fixture row, while GLEAM needs an obs4REF equivalent; the issue is done when both blocked references can be ingested and the related ILAMB migration checks pass.

Written by the indexing model from the issue text.