Expire the index cache on each provider's publishing cadence

The AMOC cache I added had no expiry, so it would have gone stale silently as
RAPID published new releases. Fixing that properly meant giving every index the
same treatment rather than special-casing one.

Each entry now records how its source updates: monthly for the NOAA indices,
annual for RAPID, none for LCR, which was published with a paper and ends at
2014. That cadence sets how long a cached copy may be reused - a week, a month,
or forever - so a living index re-downloads on its own and a finished one is
never re-fetched pointlessly.

Three things back it up.

The staleness check is on the data rather than the cache, because a fresh
download of a stale file is still stale. If a series ends further behind the
present than its provider's own lag, that is reported with the command to force
a refresh, and the note that if refreshing changes nothing the provider has not
published either.

A failed download with a cached copy on disk returns the cached copy and warns,
rather than erroring. A provider being briefly down should not become an outage
here. Saying so is what keeps it from being mistaken for current.

climate_index_status() reports what is cached and what is due without
downloading anything, so it is safe offline. refresh_climate_index() forces a
re-fetch and skips the indices that cannot change.

Downloading also moves out of the NetCDF reader into cached_index_file(), which
every format now shares. The text indices were previously re-downloaded on every
call and are now cached too. That made the mocking in the AMOC tests
unnecessary: read_rapid_netcdf() takes a path, so those tests pass a fixture
directly instead of intercepting the cache.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: chross22

NAMESPACE

@@ -2,11 +2,13 @@
 
 S3method(print,datamatch_dictionary)
 S3method(print,datamatch_index_dictionary)
+S3method(print,datamatch_index_status)
 export(accessEnvDat)
 export(as_markdown)
 export(attach_bathymetry)
 export(attach_climate_index)
 export(bathymetry_variables)
+export(climate_index_status)
 export(climate_indices)
 export(copernicus_variables)
 export(covariate_columns)
@@ -24,6 +26,7 @@ export(plot_env)
 export(plot_matched)
 export(plot_series)
 export(product_url)
+export(refresh_climate_index)
 export(upscale_grid)
 export(upscale_time)
 export(variable_dataset)