Fix Firefox: wheels, dark shadows, theme glass, smooth jumps, tri-state

Found by driving Firefox 158 with real pointer, wheel and key input.

- Time picker wheels used padding to reach the last rows. Firefox leaves
  bottom padding out of the scrollable area, so the last rows were
  unreachable and the 24H/AM/PM wheel could not scroll at all. Spacer
  pseudo-elements replace the padding.
- Wheels can now be dragged with a mouse or pen, with inertia, a rubber-band
  at the ends and a spring back. Touch is left to the platform.
- Theme changes no longer use a page-wide view transition. The snapshot
  could not blur what sits behind frosted glass, so every frosted surface
  went clear for the length of the cross-fade.
- Dark shadows were invisible: black on a near-black canvas, with a deep
  negative spread tucking the blur under the element. The dark canvas is
  lifted, the shadows are stronger and the spreads milder.
- Same-page links now travel (scroll-behavior: smooth, motion permitting).
- data-nv-tristate: a checkbox that cycles unchecked, checked, partial; the
  browser can display a partial state but a click can never choose it.
- Button groups show their pressed segment.

GUIDELINES.md gains the cross-engine rules these defects taught, and makes
Firefox the first browser to verify in.

check.py, build.py and build.py --check pass.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

index.html

@@ -768,7 +768,7 @@
             Weekly digest
           </label>
           <label class="nv-choice">
-            <input class="nv-checkbox" type="checkbox" id="f-indeterminate">
+            <input class="nv-checkbox" type="checkbox" id="f-indeterminate" data-nv-tristate="partial">
             Partial selection
           </label>
           <label class="nv-choice">
@@ -1527,7 +1527,6 @@
     addEventListener("scroll", onScroll, { passive: true });
     onScroll();
 
-    document.getElementById("f-indeterminate").indeterminate = true;
 
     /* Build the semantic color list from the tokens themselves, so the page
        cannot show a role the system does not actually define. */