feat(ui): a scrollbar held where it was taken, a popover that keeps the mouse, easing that lands

- the scrollbar is .ui-scrollbar with a .ui-thumb, styled by the default sheet or a theme; a press
  on the thumb holds it at the point it was taken and the pointer moves it in proportion, a press on
  the track jumps the thumb's middle there and holds it, and neither presses what is under the bar
  (it used to centre the thumb on the pointer and press the row beneath as well);
- while a popover is up only scroll boxes inside it take the pointer, and a press outside it that
  closes it forgets any press in progress;
- a transition ends exactly on its value (it stopped a rounding error short, at every frame rate).

Tests: ui_popover_mouse, ui_scrollbar, ui_ease (60/120/180/240 Hz).

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

examples/library/ui_popover_mouse_parts/Grid.lss

@@ -0,0 +1,3 @@
+.page { width: 300px; height: 300px }
+.cell { width: 200px; height: 100px }
+.verbs { top: 10px; left: 10px; width: 120px }

examples/library/ui_popover_mouse_parts/Grid.ludic

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+# Grid.ludic - a grid cell with a popover of verbs laid over it, written BEFORE the cell, so the cell
+# comes later in the tree and would win any hit test that went by the order things are drawn in
+component Grid {
+  state open: bool = false
+  state opened: int = 0
+  state dropped: int = 0
+  state closed: int = 0
+}

examples/library/ui_popover_mouse_parts/Grid.xml

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+<div class="page">
+  <if test="{open}">
+    <div popover class="verbs" on-close="set open = false; set closed = closed + 1">
+      <button id="drop" on-click="set dropped = dropped + 1; set open = false">Drop</button>
+    </div>
+  </if>
+  <button id="cell" class="cell" on-click="set opened = opened + 1; set open = true">Axe</button>
+  <p>opened {opened} dropped {dropped} closed {closed} {open}</p>
+</div>