fix #374: open dropdown popups get first claim on pointer routing

Campaign OP gate 2 root cause: UiElement.HitTest walks siblings
front-to-back by z-order, so an OPEN UiMenu's extended button+popup
hit-test union was never consulted when a LATER sibling's rect overlapped
the popup area — on the Config tab every dropdown has rows below it, so
Resolution-item clicks toggled the Full Screen / VSync rows underneath
(the gate session's persisted fullscreen/vsync flips were exactly those
stolen clicks). Latent since UiMenu existed; vendor/chat menus only
worked by z-order luck.

Fix: UiMenu's open/close now registers with UiRoot (SetActivePopup /
ClearActivePopup); a registered popup gets FIRST claim on mouse-down,
scroll, and hover routing; a press outside a live popup dismisses it and
is SWALLOWED (the dismissing click must not act on what sat underneath);
hidden/detached owners self-heal the registration on the next pointer
event. UiMenu gains the IsOpen seam and a single SetOpen writer.

Also in this commit, from the same investigation:
- SilkRuntimeDisplayWindowTarget.Apply documents the fullscreen half
  honestly: IViewProperties.VideoMode is READ-ONLY, so a resolution pick
  while fullscreen cannot switch the display mode through Silk's
  abstract API — split out as #376 (native glfwSetWindowMonitor port)
  rather than half-shipping untested native interop at a gate tail.
- Gate script §OP6 step 8 re-scoped: test resolution in WINDOWED mode.

Regressed by tests/AcDream.App.Tests/UI/UiMenuPopupRoutingTests.cs —
4 tests driving the real UiRoot input path on a mounted overlapping
tree, with an in-test overlap CONTROL click so the popup assertions
cannot pass vacuously (the #372 lesson: only mount+drive-input tests
catch this class; every fixture-conformance test stayed green through
this bug).

Full Release suite: 13,081 passed / 4 skipped / 0 failed.

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

docs/research/2026-08-11-campaign-op-test-script.md

@@ -603,8 +603,16 @@ should show no caption.
 ### Live rows — display
 
 8. **Open the Resolution menu (Graphics Options, first row) and pick a
-   different resolution.** The window should resize IMMEDIATELY, live, no
-   restart needed.
+   different resolution — IN WINDOWED MODE.** The window should resize
+   IMMEDIATELY, live, no restart needed. **Gate-2 re-test note (#374):**
+   the first gate's failure here was a client-wide dropdown routing bug —
+   clicks on an open popup's items landed on the rows UNDERNEATH it (your
+   session's Full Screen and VSync flips were those stolen clicks). Fixed;
+   dropdown items now win while a popup is open, and a click outside an
+   open popup dismisses it without acting on what's below. While
+   FULLSCREEN a resolution pick cannot switch the display mode yet
+   (Silk API limit, #376) — it applies on the next return to windowed;
+   do not report that as a step-8 failure.
 9. **Toggle "Full Screen".** The window should switch between windowed and
    fullscreen IMMEDIATELY, live.
 10. **Toggle "Sync To Refresh" (VSync) and drag the "Field of View"