fix(indexexport): read an index older than the binary

`maintain-index` failed with

    indexexport: copy rows: SQL logic error: no such column: total_tracks

three minutes into the one job that owns the ~205 GB checkpoint and
publishes the catalog every user downloads.

The cause is the exception that keeps that checkpoint alive. The job's
/cache is a real YJ_HOME that survives between runs, so its
explore_index is classified Cache and is deliberately *not* dropped and
recreated by cmd/indexbuild's schema repair -- which means a column
added to the schema afterwards is absent from it. total_tracks arrived
with the album-completeness work; the exporter selected it regardless.

The fix is the rule the importing side already follows.
artifactHasTotals exists because "adding a column to the importer's
SELECT is how you break every artifact already published"; the mirror
image, reading an index older than the binary, had no such guard.
sourceColumns asks pragma_table_info and selects a literal 0 when the
column is absent -- which is what that column already means by "the
catalog does not say", and what the app renders as unknown rather than
as incomplete. The artifact keeps every column, so an importer needs no
second shape.

The test reproduces the failure symptom first: with the fix removed it
fails with the CI message verbatim. Its own first version proved
nothing, though, and that is worth the comment it now carries --
`strings.Replace(catalogColumns, "total_tracks, ", …)` matches nothing,
because the list is formatted across lines and the name is followed by
a newline, so the "old" index was built with every current column.

.planning/NOTES.md

@@ -2983,3 +2983,54 @@ terminates the tagged template, and the failure arrives as
 `Expected "]" but found "wa"` from the CSS parser, at a line number in
 the *comment*. `make css-check` exists for this and named it
 immediately.
+
+## The index artifact could not be exported, and the reason is a rule this repo already had (2026-08-16)
+
+`maintain-index` failed on an unrelated push:
+
+```
+indexexport: copy rows: SQL logic error: no such column: total_tracks (1)
+```
+
+Three minutes in, on the one job that owns the ~205 GB checkpoint and
+publishes the catalog every user downloads.
+
+**The cause is the exception that keeps that checkpoint alive.** The
+index job's `/cache` is a real `YJ_HOME` that survives between runs, so
+`explore_index` there is classified `Cache` and is deliberately *not*
+dropped and recreated by `cmd/indexbuild`'s schema repair
+(`staleschema.go`). A column added to the schema afterwards is
+therefore simply absent from that database — and `total_tracks` was
+added by the album-completeness work. The exporter selected it anyway.
+
+**The fix is the rule the importer already follows.**
+`artifactHasTotals()` exists precisely because "adding a column to the
+importer's SELECT is how you break every artifact already published";
+the mirror image — *reading* an index older than the binary — had no
+such guard. `sourceColumns()` asks
+`pragma_table_info('explore_index', 'main')` and selects a literal `0`
+when the column is not there, which is what the column already means by
+"the catalog does not say" and what the app already renders as unknown
+rather than as incomplete. The destination keeps every column, so an
+importer needs no second shape.
+
+So the pattern generalises, and is worth stating once: **any query that
+crosses a version boundary in either direction asks the schema rather
+than trusting it.** There are now three of these — `artifactStoresText`
+(encoding), `artifactHasTotals` (import), `sourceColumns` (export).
+
+Two things about the test are worth keeping.
+
+It reproduces the failure **symptom first**: with the fix removed it
+fails with the CI message verbatim, `copy rows: SQL logic error: no
+such column: total_tracks (1)`. That was checked, not assumed.
+
+And its first version silently proved nothing. `oldColumns` was
+`strings.Replace(catalogColumns, "total_tracks, ", "", 1)` — which
+matches *nothing*, because the list is formatted across lines and the
+name is followed by a newline rather than a space. So the "old" index
+had every current column, the probe correctly said so, and the only
+reason this was caught is that the assertion about the probe ran before
+the assertion about the export. A fixture built by string surgery on a
+formatted constant needs to be whitespace-independent; it filters the
+list now.