FIX: write disk cache files atomically

[redeboer]

Closes #506

🐛 Bug fixes

• Cache files are now written to a temporary file in the same directory and moved into place with os.replace(), so a concurrent reader never observes a half-written file.
• A cache file that is empty or otherwise unreadable is now ignored with a warning and recomputed, instead of raising from pickle.load().
• A failing cache write (full or read-only cache directory, or a locked destination file) no longer discards an already-computed result: it is logged as a warning and the result is returned uncached.

🖱️ Developer experience

• The Sphinx linkcheck builder now allows 60 seconds per link instead of the 30-second default, which was causing spurious CI failures on slow-responding hosts such as zenodo.org.

Squash commit messages

* DX: increase Sphinx linkcheck timeout
* FIX: keep computed result when cache write fails