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.

.gitignore

@@ -7,6 +7,10 @@ public/assets/
 # the git pull at the top of every build tick
 notebooks-export/
 public/nb/
+# the reading map is the same kind of artifact: library-rag-export.service regenerates it on
+# cyrion whenever the corpus or its located highlights move. It was tracked until 2026-08-14,
+# which is why automating the export needed this line first.
+public/reading_umap.json
 .build_state
 .build.lock
 .notebooks.lock