Document the bugs found and how they were fixed

[chross22]

Adds NEWS.md, which the package did not have, and a Troubleshooting section to the README.

NEWS.md

Leads with the layer-ordering bug, because that one can affect results already in hand rather than only future runs. It records:

• what was affected (any download of 2+ variables whose requested order differed from alphabetical code order; single-variable fetches never were);
• how to recognise it in existing output — a column whose values sit in the wrong range for its name, salinity near 32 in an SST column;
• that the cached NetCDF files were always correct, so only naming was wrong and a re-fetch is enough.

Also records the two other fixes, and the cache move — the one change that silently alters behaviour on an existing machine, with the one-liner to point at the old directory.

README troubleshooting

Keyed to the literal error text, so a message can be pasted and found. Covers the client-not-found case, the different-datasets refusal (which is expected behaviour, not a fault), missing variables, depth ranges, and download failures.

It also covers two messages from before today's fixes, since anyone on an older install will hit those rather than the current ones — including "values in the wrong range for their column name", which has no error message at all.

One detail worth noting

The version heading is # datamatch 0.0.0.9000, not # datamatch (development version). R's NEWS.md parser wants a version it can parse; the usethis-style heading produces a No news entries found NOTE from R CMD check. Verified both ways — with the usethis heading the check was 1 NOTE, and with the version number it is clean.

Verification

• R CMD check — Status: OK.
• tools:::.build_news_db_from_package_NEWS_md() parses the file: 3 entries under version 0.0.0.9000, categories Bug fixes / Breaking changes / New features.
• All README anchors re-checked, including the new TOC entry; none broken.
• Full suite green.

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