docs(booth.html): name the .rail cross-file contract at the selector that depends on it

The SVOS retheme makes .rail load-bearing in two files owned by two different
agents: this template's grid-cursor start, and base.html's --rail-h measuring
script that publishes the rail's height for scroll-margin-top (the rail wraps,
so no CSS number can know it).

Neither breaks loudly if it is renamed. Ours starts the cursor one tile too
high; theirs falls back to a fixed guess. design-dev's sheet carries the mirror
of this note above the .rail rule, so the coupling is documented from both ends
rather than from whichever side happened to notice.

booth/templates/booth.html

@@ -411,6 +411,16 @@
      The first tile whose bottom edge clears the sticky rail is the one the
      reader is looking at, so that is where the cursor picks up. */
   function fromViewport() {
+    /* ⚠ `.rail` IS A CROSS-FILE CONTRACT, read by two scripts in two files
+       owned by two different agents: this one, and the --rail-h measuring
+       script in base.html that publishes the rail's height for
+       `scroll-margin-top` (the rail wraps, so no CSS number can know it).
+       RENAMING IT BREAKS BOTH, and neither breaks loudly — this one falls back
+       to treating the viewport top as the boundary and starts the cursor one
+       tile too high; that one falls back to a fixed guess. base.html carries
+       the mirror of this note above the `.rail` rule. Agreed with design-dev
+       2026-09-23 during the SVOS retheme, which is the change that made the
+       selector load-bearing in two places instead of one. */
     var t = tiles(), rail = document.querySelector('.rail');
     var top = rail ? rail.getBoundingClientRect().bottom : 0;
     for (var i = 0; i < t.length; i++) {