fix(explore): merge the catalog artifact in its own mbid encoding

The prebuilt catalog never merged. `mergeArtifactRows` positions itself
with `WHERE mbid > ? ORDER BY mbid LIMIT 1 OFFSET ?` against the
attached artifact, and it bound that cursor as a Go `string` while
`cmd/indexexport` publishes `explore_index.mbid` as 16 raw bytes — the
storage change that took the table from 677 MB to 389 MB.

SQLite does not coerce between TEXT and BLOB and orders every blob after
every text value, so against a byte column the predicate was not wrong
but unconditional: `mbid > <text>` matched the whole artifact, so the
bound the walk looked up was the same row every time and the cursor
never advanced, and `mbid <= <text>` matched nothing, so no batch
merged. No error, no rows, no state change — a fresh install sat at "0
of 1,077,893 rows" burning a core indefinitely, which is what it did
here for a day, while Explore showed only the rows the library scan and
the lazy artist enrichment had produced and popularity for none of the
catalog.

The cursor is now an `artifactKey`, typed to the encoding
`artifactStoresText` reports for the file it is attached to, so the
comparison is made in the same type as the column it is made against.
Two things guard the class rather than the instance: a nil key binds as
an empty value instead of SQL NULL, because `mbid > NULL` agrees with
nothing and would import nothing just as silently; and the walk returns
an error when its bound does not strictly advance, because the failure
here is silence and the next one should be a failed job with a reason.

It was never caught because the fixture that guards the walk writes the
old text encoding, and the only compact fixture is a single row — below
`artifactMergeBatch`, so the bound query never ran at all. The walk is
now covered on both encodings, across several batch boundaries.

Closes #258

CLAUDE.md

@@ -845,6 +845,22 @@ that is quietly empty.
   top-N, exact match, FTS search, popularity batch, the CAA map — and
   asserts each returns something with a dashed id. A missed conversion
   site shows up there and essentially nowhere else.
+- **A comparison is typed on *both* sides, and a parameter is the half
+  that gets forgotten.** The paragraph above is about a literal; the
+  artifact merge positioned its batch walk with a Go `string` cursor
+  against the artifact's byte column, and SQLite answered rather than
+  complained: `mbid > ?` with a text key is true of every row, so the
+  bound the walk looked up was the same every time and the cursor
+  never advanced, while `mbid <= ?` is false of every row, so no batch
+  merged at all. The import looped indefinitely at 100% CPU behind a
+  progress bar reading "0 of 1,077,893 rows", merged nothing and
+  raised nothing (#258). Nothing caught it because the fixture that
+  guards the walk writes the old text form and the only compact one is
+  a single row — below `artifactMergeBatch`, so the bound query never
+  ran. `artifactKey` types the cursor to the artifact's own encoding
+  now, and the walk fails loudly when its bound does not strictly
+  advance, because the failure mode here is silence rather than a
+  wrong answer.
 
 **The artifact is read in either encoding.** A published artifact
 carries whichever form the exporter that built it used, and there is one