fix: explain when windows cannot fit a layout

[culpen90]

When an app will not shrink a window enough for a snap layout, show a brief “Window size limited” message on the destination display after the final resize attempt. This explains overlaps such as a window with a 600-point minimum width placed in a 504-point third. The message dismisses automatically without taking focus.

Ignore rounding differences and position-only actions, and discard superseded cross-display retries so they cannot restore stale geometry or feedback. Document the app-imposed size limit and available layout workarounds.

fixes #522.

Validation:

• All 320 tests passed on macOS 26.6.2, including 10 new regression tests for size limits, rounding, invalid frames, and cross-display retries.
• Native warning rendering, repeated-show dismissal timing, and foreground app preservation checked.
• Safari-specific behavior was not manually tested.

[rxhanson]

Thanks! A couple of minor feedback points:

• It'd be nice to have a defaults setting to allow this to be turned off, since there's the potential that it would be shown a lot for some users depending on their workflow and it could be undesirable.
• It could use a bit better wording. Maybe something like this:
  Minimum window size reached Unable to resize window smaller. Windows may overlap.

[rxhanson]

I went ahead and adjusted the wording and will roll in a default for this. Thanks again!