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/db/schema.sql

@@ -675,7 +675,7 @@ CREATE TABLE IF NOT EXISTS shard_atlas_pending (
 -- because the manifest reports them before the pixels are fetched and a screen
 -- that lists what WOULD be imported needs them then.
 CREATE TABLE IF NOT EXISTS shard_assets (
-  asset_key   VARCHAR(191) NOT NULL PRIMARY KEY,  -- §5's key: `body/34/a0`
+  asset_key   VARCHAR(191) NOT NULL PRIMARY KEY,  -- §5's key: `body/34/a0`, `body/820/a23`
   family      VARCHAR(24)  NOT NULL DEFAULT 'body',
   sha256      CHAR(64)     NOT NULL,
   bytes       INT          NOT NULL DEFAULT 0,
@@ -708,6 +708,21 @@ CREATE TABLE IF NOT EXISTS shard_assets (
 -- same test and costs one re-fetch.
 ALTER TABLE shard_assets ADD COLUMN IF NOT EXISTS catalog VARCHAR(32) NULL;
 
+-- Which action a body's thumbnail came from (§11.2, phase 6).
+--
+-- The catalogue is still one row per body and still a first frame; what changed
+-- is that a body with no art at action 0 is catalogued at the first action that
+-- has any, and the key says so — `body/820/a23` is a horse whose action 0 is
+-- empty. 73 of a stock client's bodies are in that state, and they rendered as
+-- text on the bestiary until this phase looked one action further.
+--
+-- It is stored rather than parsed back out of the key because the atlas join
+-- needs it in SQL, and re-deriving it there with SUBSTRING_INDEX would put a
+-- second, weaker parser of §5's key scheme in the schema. NULL means a row
+-- written before this column existed, which is action 0 by definition — every
+-- key the catalogue had then ended in `a0`.
+ALTER TABLE shard_assets ADD COLUMN IF NOT EXISTS action TINYINT NULL;
+
 -- Slug → body id, as the shard itself answered it (§8).
 --
 -- **Deliberately NOT a column on `shard_spawn_creatures`.** That table is