Open menu row menus flush under the bar (#143)

Material3's DropdownMenu clamps every popup to 48dp below the window's
top edge (MenuVerticalMargin in its position provider); the margin is
applied after the offset parameter and the provider is not replaceable,
so the in-window menu bar's menus all opened with a visible gap hanging
below the bar. Replace them with a plain focusable Popup in MenuDefaults
styling and an own position provider that puts the menu directly under
its anchor, flipping above only when there is no room below. The anchor
box takes the full bar height so the menu starts at the bar's bottom
edge, not at the centred title text.


Claude-Session: https://claude.ai/code/session_01BegdsGSNNxKR6VPNPNvfoc

Co-authored-by: Claude <noreply@anthropic.com>

docs/PROJECT_CONTEXT.md

@@ -317,6 +317,14 @@ section records only the durable architecture and operational boundaries.
 - **A column's `id` is never translated.** It is what the remembered column widths and order
   are keyed on; translating it would read an operator's stored layout under names that no
   longer match.
+- **The menu row's menus are a hand-rolled popup, not Material3's `DropdownMenu`** (release
+  1.50): M3's position provider refuses any position closer than 48 dp to the window's top
+  edge (`MenuVerticalMargin`), the margin is applied after the `offset` parameter and the
+  provider is not replaceable — so every menu of a bar at the top of the window opened with a
+  visible gap below it. `MenuRowPopupPositionProvider` plus a plain `Popup`/`Surface` in
+  `MenuDefaults` styling opens flush under the bar; outside-click and Escape dismissal come
+  with a focusable skiko popup for free, only the expand animation is gone. The combo boxes
+  keep the stock `DropdownMenu`: they never sit at the window edge.
 
 - **The palette has three sources, and the later one wins role by role** (release 1.50): the
   shipped stylesheet from the classpath, then the profile's own stylesheet file if present,