Left-align menu entries, add a Dropdown field, scrollable and searchable menus

Menu entries are left-aligned and live in a ScrollContainer capped at 320 px, so a
long list (the parent picker holds every object of the scene) scrolls instead of
running off the screen. `Menu(items, { searchable: true })` puts a filter box above
the scroll area; Enter takes the first match. Filtering forces a relayout because
two queries with the same hit count leave the height unchanged, which would keep
the stale row positions.

Fields that open a menu are a new `Dropdown` widget: the value sits flush left and
the chevron is drawn flush right instead of being written into the label. A menu
opened below a field is never narrower than that field.

New debug hooks `overlayTexts()` / `overlayRect()` make the popup contents
assertable; e2e step 11 covers the cap, the scrolling and the search box.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/widgets/Dropdown.ts

@@ -0,0 +1,38 @@
+import { Graphics } from 'pixi.js';
+import { Theme } from '../Theme';
+import type { Size } from '../Widget';
+import { Button, type ButtonOptions } from './Button';
+
+/** Space reserved at the right edge for the chevron. */
+const CHEVRON_SLOT = 16;
+
+/**
+ * A button that opens a menu: value flush left, chevron flush right. The chevron is drawn
+ * rather than written into the label, so the label stays the plain value – which keeps it
+ * readable, lets it be compared against the menu entries, and stays crisp at any size.
+ */
+export class Dropdown extends Button {
+  private readonly chevron = new Graphics();
+
+  constructor(label: string, options: ButtonOptions = {}) {
+    super(label, { ...options, align: 'left' });
+    this.addChild(this.chevron);
+  }
+
+  override measure(): Size {
+    const size = super.measure();
+    return { w: size.w + CHEVRON_SLOT, h: size.h };
+  }
+
+  override redraw(): void {
+    super.redraw();
+    const x = this.w - CHEVRON_SLOT / 2 - Theme.padding / 2;
+    const y = this.h / 2;
+    this.chevron
+      .clear()
+      .moveTo(x - 4, y - 2)
+      .lineTo(x + 4, y - 2)
+      .lineTo(x, y + 3)
+      .fill(this.onAccent ? Theme.colors.accentText : Theme.colors.textMuted);
+  }
+}