Support AIMIP IC and forcing generation for a different ERA5 build

[brianhenn]

The AIMIP IC and forcing scripts assumed one ERA5 build and one model's variable set.
Generating inputs for a model trained on a later ERA5 build required editing the scripts,
and the prepend step stored static variables once per timestep.

HGTsfc, DSWRFtoa and land_fraction are read from an ACE ERA5 zarr rather than from
the AIMIP protocol data, and they differ between ERA5 builds — the March 2026 pipeline
rewrite changed the regridding package and some data sources, moving HGTsfc by tens of
metres in mountainous terrain. The source is now selectable so the forcing can match the
build a model was trained on.

Changes:

• create_aimip_ic_datasets.py: --include-near-surface adds TMP2m/Q2m/UGRD10m/VGRD10m
  for models carrying them as prognostics. sel(time=[...]) keeps time a length-1
  dimension, which get_initial_condition requires.

• interpolate_aimip_forcing.py: --extension-start, --repeat-source-start and
  --repeat-source-end make the synthetic insolation extension opt-in. Previously 2023
  onward was always synthesized, which is wrong for a source that already spans the window.
  Without the flag, a source ending before --end-time now raises instead of silently
  truncating.

• prepend_first_timestep_forcing.py: pass data_vars="minimal", coords="minimal",
  compat="override" to xr.concat, so variables without a time dimension are not
  broadcast along it. The data loader handles static variables natively; broadcasting only
  inflates the store and slows loading.

• encoding.py: skip 0-d variables in set_shards_chunks. zarr rejects empty chunk/shard
  tuples, which scalars hit once they survive the concat above. Not reachable from the
  6-variable AIMIP forcing, but is from the 73-variable combined forcing, which carries 18
  ak_*/bk_* scalars.

• Makefile: IC_EXTRA_ARGS and FORCING_EXTRA_ARGS passthroughs; OUTPUT_ZARR_NAME and
  MODIFIED_FORCING_NAME become ?= so a regeneration cannot overwrite existing datasets.

• README.md: documents the ERA5-build dependency and the new options.

• Tests added

• If dependencies changed, "deps only" image rebuilt and "latest_deps_only_image.txt" file updated