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>

REFERENCE.md

@@ -527,6 +527,7 @@ everything still renders — state changes simply arrive instead of travelling.
 | `data-nv-calendar-step` | button in a calendar | `-1` or `1`: months to move. |
 | `data-nv-timepicker` | `.nv-timepicker` | Marks a time picker. Also `data-nv-value` (`HH:MM`), `-hours` (`24` default / `12`), `-step` (minutes), `-name` (hidden form field), `-format-toggle` (adds the 24H / AM / PM wheel). |
 | `data-nv-picker-label` | child of a picker trigger | Receives the chosen option's text; default is the trigger itself. |
+| `data-nv-tristate` | `input[type=checkbox]` | Click cycles unchecked → checked → partial. `="partial"` starts partial. Exposes `data-nv-state` (`off`/`on`/`partial`). |
 | `data-nv-scroller` | `.nv-scroller-frame` | Wires its `[data-nv-scroll="prev"\|"next"]` controls. |
 | `data-scrolled` | `.nv-nav` | Set by the application on scroll; gives the bar its shadow. |
 | `data-nv-direction` | `<html>` | Set during a transition by `nv.transition`. |