fix: prevent partial preformatted cache after interrupted update

[benthamite]

Summary

citar-cache--preformat-bibliography previously updated the bib's preformatted hash in place via clrhash followed by maphash + puthash. If the refill is interrupted partway through — for example, C-g during an idle-time update of a large bibliography — the preformatted hash is left with only a fraction of the entries. Because citar-cache--update-bibliography has already set props before calling the preformatter, subsequent cache-up-to-date checks see "no changes needed" and the partial preformatted cache persists indefinitely. citar--format-candidates then errors with

No preformatted candidate string: <KEY>

on any entry that didn't make it into the hash before the interruption, breaking citar-insert-citation, citar-select-refs, etc.

I hit this against a ~15k-entry bibliography: after some earlier interruption, the bib's entries cache had 14,662 keys but the preformatted cache only had 7,986. The partial state then persisted across Emacs sessions until citar-cache--preformat-bibliography was called manually.

Fix

Build the new preformatted hash in a fresh local hash table and replace the bib slot via setf only after maphash completes. If the maphash is interrupted, the setf never runs and the existing (valid) preformatted hash is retained.

This mirrors the atomicity pattern introduced in #882 for the entries slot.

Test plan

• Exercised the patched function against the affected bibliography; preformatted is fully populated (14,662/14,662) and the previously missing key resolves.
• Verified the hash is a fresh object (eq returns nil against the old slot value), confirming the atomic swap.
• Byte-compiles cleanly.