#### Brief overview of PR changes/additions
- MXP `<FRAME>` windows were positioned against the whole main window,
so an edge-aligned frame landed on top of space a package had reserved
with `setBorderRight()` and friends. They now lay out inside the area
the user borders leave.
- Frames are repositioned when those borders or the window size change,
instead of staying where they were first put.
- Frames in a window of their own (`EXTERNAL`) are left alone by that
repositioning.
#### Motivation for adding to Mudlet
With the base UI installed, a game using MXP frames drew its frames on
top of the UI panel instead of beside it.
#### Other info (issues closed, discussion etc)
Fixes#9698
New functional test `MxpFramePlacementTest` (16 cases) covers each edge,
border changes, window resizes, nested frames, external frames, and a
Geyser `Adjustable.Container` attached to the right border, which is how
the base UI reserves its space. 8 of the cases fail on `development`.
**Test case:** With the base UI installed, connect to
`eden-test.rpgframework.de 4000` - the MXP frames sit beside the UI
panel rather than under it.
Assisted-by: Claude:claude-opus-5
