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.

deploy/auto-build.sh

@@ -30,6 +30,9 @@ sig=$({
     git diff
     find "$VAULT" -name .obsidian -prune -o -type f -printf '%T@ %p\n' | sort
     find "$REPO/notebooks-export" -type f -printf '%T@ %p\n' 2>/dev/null | sort
+    # same deal for the reading map: gitignored, written by library-rag-export.service on its
+    # own timer, so git status/diff above cannot see it change
+    find "$REPO/public/reading_umap.json" -printf '%T@ %p\n' 2>/dev/null | sort
 } | sha256sum | cut -d' ' -f1)
 
 if [ "$FORCE" -eq 0 ] && [ -f "$STATE" ] && [ "$(cat "$STATE")" = "$sig" ]; then