Allow NUMERICALEARTH_DATA_DIRECTORY to override the Scratch.jl cache

[glwagner]

Summary

Adds an opt-in NUMERICALEARTH_DATA_DIRECTORY environment variable that lets users redirect where downloaded datasets are cached, as an alternative to the default Scratch.jl space.

Scratch.jl ties its scratch space to the active Julia depot and offers no per-package env-var override — the only knob is JULIA_DEPOT_PATH, which relocates everything (installed packages, precompile caches, all scratchspaces). This is too blunt for the common cases:

• HPC clusters where the depot lives on a small / quota-limited filesystem (e.g. $HOME) but a large scratch filesystem is available for data;
• sharing one cache of large datasets (GEBCO ~8 GB, IBCAO ~25 GB, ECCO, ...) across depots or users.

What changed

• New helper DataWrangling.download_cache(key):
  • default → @get_scratch!(key) (unchanged behavior);
  • if NUMERICALEARTH_DATA_DIRECTORY is set → mkpath(joinpath(ENV[...], key)).
• All 15 dataset/bathymetry caches now resolve through this helper in their __init__ instead of calling @get_scratch! directly, which also removes the duplicated Scratch import from each submodule.
• Per-key subdirectories are preserved (e.g. ECCO/v4, WOA/annual), and the per-Metadata dir keyword still overrides everything for individual datasets.

Because the helper lives in DataWrangling and all submodules share NumericalEarth as their module root, @get_scratch! still resolves to the same package UUID — so existing default caches keep their current on-disk location (verified: ~/.julia/scratchspaces/904d977b-.../JRA55).

Verification

• Env-var path: default_download_directory(...) lands under the configured directory (incl. ECCO/v4 subdir). ✅
• Default path: unchanged Scratch location. ✅
• All ExplicitImports QA checks (check_no_implicit_imports, check_no_stale_explicit_imports, check_all_explicit_imports_via_owners, check_all_qualified_accesses_via_owners, check_no_self_qualified_accesses) return nothing. ✅
• New testset in test/test_metadata.jl covering both the fallback and the env-var override. ✅

Docs

Added a "Where data is cached" section to docs/src/Metadata/metadata_overview.md.

cc @monsieuralok

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/DataWrangling/DataWrangling.jl	75.00%	1 Missing ⚠️

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