docs(probe): the probe covers the index only, and says so now

The docstring claimed a no-argument run probes 'the booth index and
every booth linked from it'. main() probes argv[1:] or the default URL
and follows nothing — so a coverage claim that reads as 26 pages has
always been one. A probe that overstates its reach is worse than one
that states a small reach honestly, because this is the instrument
standing in for a class of bug the test suite structurally cannot see.

Also records the zsh trap that hid it: an unquoted $URLS holding twelve
space-separated URLs arrives as ONE argument, and the probe cheerfully
reports '2 page(s)' while covering two.

scripts/layout-probe.py

@@ -19,7 +19,14 @@ USAGE
     <a python with playwright> scripts/layout-probe.py [URL ...]
 
 Exits 0 if every control is hittable, 1 if any is occluded. No arguments
-probes the booth index and every booth linked from it.
+probes the INDEX ONLY — it does not follow booth links, and the docstring
+claimed it did until 2026-09-22. Pass booth URLs explicitly to cover them:
+
+    scripts/layout-probe.py http://10.100.10.50:8090/{,b/my-run/}
+
+⚠ In zsh an unquoted `$URLS` does NOT word-split, so a variable holding
+several URLs arrives as ONE argument and the probe silently reports
+"2 page(s)" while covering two. Use an array and `"${URLS[@]}"`.
 """
 import sys
 from playwright.sync_api import sync_playwright