Start the fade-in only after the menu surface has been submitted

[danielchalmers]

Fixes #74.

Mechanism (measured, not guessed)

The OS hit-tests a layered window against its last submitted surface, and the first submission after Window.Show() lags by ~120-150 ms on a cold open (~30-60 ms warm). The fade animation started immediately on show, so its clock ran against a window that was still invisible and click-through. Probing WindowFromPoint (the same OS path real clicks take) against the fade timeline proved the point:

• With the fade artificially stretched to 4 s, the window became clickable at a fixed ~150 ms regardless of opacity (~11% at that moment).
• At the normal 220 ms fade, that same ~150 ms meant the menu appeared mid-animation at ~85-95% opacity (the fade was effectively invisible) and clicks in the gap fell through to the window beneath - which then activated, deactivated the menu, and dismissed it.

Fix

Defer BeginFadeIn by two CompositionTarget.Rendering ticks after ShowMenu: one tick renders the shown surface, the second guarantees it was submitted. Event-driven, no timers. A version counter cancels a pending fade-in if the menu hides first.

Measured result (same probe, fixed build)

Open	Before: visible / hittable	After: visible / hittable
#1 (cold)	+115 ms / +122 ms	+70 ms / +63 ms
#2	+54 ms / +50 ms	+54 ms / +46 ms
#3	+58 ms / +32 ms	+50 ms / +45 ms

The menu is now hittable before the first visible pixel on every open, and the full 0-to-1 fade actually renders instead of popping in near-full opacity. Clicking a slice the instant it appears executes it (verified 3/3 opens, cold included).

Verification

• Hit-test/visibility probe as above, plus the full user-level smoke (mouse click, keyboard activation, flick, drag-reorder with settings persistence): all pass.
• dotnet test: 148/148.
• Pie interior pixel diff vs main: 3 of 121,905 pixels within anti-aliasing tolerance - no visual change to the rendered menu.

Residual (inherent to layered windows): a truly blind click in the first ~50-70 ms - before anything is on screen - still lands beneath the menu; that window shrank from ~150 ms and can't reach zero because a surface cannot be hit-tested before it exists.