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>

niveus/base.css

@@ -134,6 +134,13 @@ textarea {
   border-radius: var(--nv-radius-xs);
 }
 
+/* A link to a place on the same page travels there instead of cutting. */
+@media (prefers-reduced-motion: no-preference) {
+  html {
+    scroll-behavior: smooth;
+  }
+}
+
 /* Whatever the browser draws itself — a bare checkbox, a progress bar, the
    text caret, a date field's glyphs — takes the brand, so no native colour
    survives next to the system's own. */