feat(assets): a creature's picture is whichever action has one (Phase 6)

The shard's catalogue can now answer for 73 bodies it used to report absent —
they have no art at action 0 and real art at a later one, and their key says
which (`body/820/a23` is a horse). This side stores that action and stops
assuming `a0` anywhere.

The atlas join is the part that mattered. It read

  a.asset_key = CONCAT('body/', b.body, '/a0')

which would have silently dropped exactly the creatures this phase adds. It now
reads the row's own action, with COALESCE for rows written before the column
existed — a NULL inside CONCAT makes the whole comparison NULL, which would have
taken every portrait off the site on upgrade with the database perfectly correct
and nothing in any log. It still matches at most one row per slug: a deeper key
(`body/820/a23/f4`) does not equal the catalogue key.

Verified against a real MariaDB with the live shard's own 1,095-row manifest: the
ALTER applies to an installed-shape table and is idempotent, the horse joins to
its a23 picture, a pre-phase-6 NULL-action row keeps its portrait, and a stored
frame key does not become a second candidate.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

server/model/shardAssets/shardItemArt.model.js

@@ -13,7 +13,7 @@ const log = require('../../core').logger('shardItemArt')
 // ── Why this is not the body catalogue with a different prefix ─────────
 //
 // The bestiary wants every creature, so phase 3 imports a SET: walk a manifest,
-// diff the hashes, fetch what moved. That works because the set is 1,022 rows
+// diff the hashes, fetch what moved. That works because the set is 1,095 rows
 // and one megabyte.
 //
 // This side has no set. The shard's client addresses 49,152 item graphics and