Count the originals on this device from the cache, not from every image

`library::local_original_count` feeds the "On this device" chip and runs
beside the rating counts on every star keystroke. On the reference library
it cost 1.3-1.4 ms best-of-50 (3 ms on a busy machine) to find 254
originals among 19,000 visible images.

It was a correlated EXISTS per visible image:

  SCAN i USING INDEX images_grid_order
  SEARCH ic EXISTS USING INTEGER PRIMARY KEY (rowid=?)

`image_cache` holds a row only for what has been fetched, so the question
is driven from it: `i.id IN (SELECT image_id FROM image_cache WHERE
tier_actual >= Original)`, which SQLite plans as the list first and a probe
of `images` by id for each entry:

  SEARCH i USING INTEGER PRIMARY KEY (rowid=?)
  LIST SUBQUERY 1
    SCAN image_cache

`image_id` is the cache's primary key, so each image is in the list at most
once and the count is the one the EXISTS gave (254). After: 0.05 ms. On a
library whose every original is cached this is as much work as before,
which is the proportion the rule asks for.

The count stays on the keystroke path: dropping it there would leave the
chip stale after a background download until something else refreshed it,
and at this cost there is nothing left to save. catalog_bench spells the
query as dr-ui does, so its copy changes with it.

core/dr-catalog/examples/catalog_bench.rs

@@ -155,8 +155,8 @@ fn local_original_count(conn: &rusqlite::Connection) -> i64 {
     conn.query_row(
         "SELECT count(*) FROM images i
           WHERE i.shadowed_by IS NULL AND i.trashed_at IS NULL
-            AND EXISTS (SELECT 1 FROM image_cache ic
-                         WHERE ic.image_id = i.id AND ic.tier_actual >= 2)",
+            AND i.id IN (SELECT ic.image_id FROM image_cache ic
+                          WHERE ic.tier_actual >= 2)",
         [],
         |r| r.get(0),
     )