Reading map: the payload is a generated artifact, not a tracked file

library-rag-export.service now regenerates reading_umap.json on cyrion
whenever the corpus or its located highlights move, writing straight into this
checkout. Keeping it tracked would defeat that: a generated file that changes
under git dirties the tree, which both flips auto-build.sh's change signature
and breaks the `git pull` at the top of every build tick — the reason
notebooks-export/ is already ignored, in that file's own words.

So it joins them: ignored, plus a find line in the signature block so the
5-minute tick still notices a new projection.

The cost is that a fresh checkout has no map until the service runs — on
cyrion that is within the hour, anywhere else never. ReadingMap.astro says so
where someone would hit it.

src/components/ReadingMap.astro

@@ -11,11 +11,16 @@
  *   src/lib/reading-map.js            ← the renderer (shared with the standalone preview
  *                                       in c0smere_devops/library-rag/umap/web)
  *   src/styles/reading-map.css        ← the chrome
- *   public/reading_umap.json          ← `uv run umap/export.py -o …`
+ *   public/reading_umap.json          ← generated; NOT in git
  *
- * ⚠️ The payload is a STATIC SNAPSHOT. Nothing on cyrion regenerates it — the 5-minute
- * auto-build rebuilds the site, not the data. New reading only reaches this map when
- * someone re-runs `export.py` and commits the result. See library-rag/umap/PLAN.md §7.
+ * The payload is regenerated on cyrion by `library-rag-export.service` (hourly, gated on a
+ * fingerprint of the corpus and the located highlights) and written straight into this
+ * checkout. It is **gitignored on purpose** — a tracked generated file dirties the tree, which
+ * flips auto-build.sh's signature and breaks the `git pull` at the top of every build tick.
+ *
+ * ⚠️ **A fresh checkout therefore has no map until that service runs.** On cyrion that is
+ * within the hour; anywhere else it is never, and the banner renders empty. Fetch a copy from
+ * garden.c0smere.net/reading_umap.json, or run `library-rag/umap/export.py` yourself.
  */
 import '../styles/reading-map.css';