test(database): the next destructive repair fails a test, not a volume

The fix for the dropped catalog pins one table in one wrong shape, which
is the failure that happened. What cost the rebuild was more general: a
destructive repair added at `database.NewDB` -- the chokepoint every
binary in this project shares -- without asking which binary it runs in.
The next one will have a different name and a different reason.

So `TestNoCacheTableIsRetiredHere` asserts the outcome instead: put every
`datamap` Cache table into a shape the schema has moved past, open the
database the way cmd/indexbuild does, and require all of them to still be
there. Driving it from `datamap.ByKind` is what makes it cover tables
nobody remembered -- flipping the policy back fails on five, including
the two artist-credit tables added the same day, where the existing test
fails on one. It asserts the rows survive too, because SQLite does an
implicit DELETE before a DROP and a repair that recreated the table would
look identical. And it accepts an error from `NewDB`, because that is the
documented trade: loud is recoverable, gone is not.

`scripts/index-cache-snapshot.sh` covers the half no test can reach. The
volume holds the only copy of a catalog that costs hours of someone
else's bandwidth to re-derive. `VACUUM INTO` rather than `cp`, since a
byte copy of a live SQLite file is a corrupt file of plausible size; the
resumable staging directory is skipped; and each snapshot is reopened and
asked for its catalog row count before anything is rotated out. A corrupt
source and an empty catalog were both exercised: each exits non-zero,
removes its own output, and leaves the previous snapshots alone.

docs/index-cache.md is the restore, and the reason to bother: a restored
snapshot resolves to `refresh` and folds in the listens since, which is
minutes against the 3-23h this rebuild has been estimating.

.planning/NOTES.md

@@ -3404,3 +3404,44 @@ Three things worth keeping from it:
   rebuilding. That is the right default, and it means the next schema
   change touching `explore_index` needs a deliberate plan for this one
   database rather than none.
+
+## Two guards for the index cache, and what each one is worth (2026-08-17)
+
+Both come out of the incident above, and they protect different halves
+of it.
+
+**`TestNoCacheTableIsRetiredHere` asserts the outcome, not the
+mechanism.** The test that shipped with the fix pins one table in one
+wrong shape, which is the failure that happened; what actually cost the
+rebuild was a destructive repair added at `database.NewDB` — the
+chokepoint every binary here shares — without asking which binary it was
+in. The next one will have a different name and a different reason. So
+this puts *every* `datamap` Cache table into a shape the schema has
+moved past, opens the database the way `cmd/indexbuild` does, and
+requires all of them to still be there.
+
+Three things it got right by being written this way. The table list is
+`datamap.ByKind(Cache)`, so the two credit tables added the same day
+were covered without anyone adding them — flipping the policy back fails
+on **five** tables including `artist_credit_part` and
+`artist_credit_ref`, where the single-table test fails on one. It
+asserts rows survive as well as the table, because SQLite does an
+implicit DELETE before a DROP and a repair that recreated the table
+would otherwise look identical. And it *accepts* an error from `NewDB`,
+because that is the trade the fix documents: loud failure instead of a
+silent day of downloading.
+
+**`scripts/index-cache-snapshot.sh` covers the half no test can.** The
+volume held the only copy of a catalog whose rebuild is hours of someone
+else's bandwidth. `VACUUM INTO` rather than `cp`, because a byte copy of
+a live SQLite file is a corrupt file of plausible size; the staging
+directory is deliberately not copied, since a build resumes without it;
+and the snapshot is reopened and asked for its catalog row count before
+any rotation happens. Both failure paths were exercised rather than
+argued: a corrupt source and an empty catalog each exit non-zero, delete
+their own output, and leave the previous snapshots in place.
+
+`docs/index-cache.md` is the restore procedure, and the number that
+makes it worth having: a restored snapshot resolves to `refresh` and
+folds in the incremental listens since — minutes, against the 3–23 h a
+rebuild was estimating.