Fix the Windows head items from the review

[SimonCropp]

The "Windows head" group from todo.md. The repros from the review are now FormsHeadTests, which drive a real form and canvas with real window messages. The image cache test moved to ImageCacheTests.

Item	Fix	What the test shows
Selection highlight and hit-test drift from the glyphs	MonoFont.Advance, the unrounded advance, places the highlight, ColumnAt and the subtitle width. The integer cell stays for the grid	Drift at column 99 is 0.2 px at 96 DPI and 0.4 px at 192. The bar at column 66 sits inside its highlight and hit-tests as 66
ImageCache never evicts	Keep drops every picture that isn't on the current screen	Nothing held after ten accepted pairs
The first window is 1100×700 device pixels	InitialClientSize scales it to the display's DPI once, when the handle is created, and keeps it inside the working area	34 characters a pane at 100% and at 200%, where it was 4
Discrete input overwrites itself within a pump	Keys, clicks and menu events are queued and drained one per frame, in order. The frame skips its wait while input is queued	Two Downs scroll 2. d then a click discards the entry that was on screen
Right-clicking the row whose menu is open	Fixed by the queue: the close and the right-click are now two frames, so the popup reopens	Popup and model agree
The thumb drag, or a move or resize, freezes the panes	New optional ILoopHooks in core. The loop hands the WinForms head a frame to run on a timer during user32's modal loops: thumb tracking, and WM_ENTERSIZEMOVE to WM_EXITSIZEMOVE	22 frames during a 566 ms drag, with the panes following the thumb before release
Lost capture mid drag	OnMouseCaptureChanged ends the drag, and so does a move with the left button up	The selection stops following, and the splitter stays put
Logoff can end before PersistOwned	ILoopHooks.SessionEnding, called from OnFormClosed for a session end, sets Closing and stages the queue inside WM_ENDSESSION	Closing is set when WM_ENDSESSION returns
ImageCache ignores the hash and holds files FileShare.Read	The hash is part of the key. FileSide reads pending files through a ReadWrite | Delete shared stream, and the cache reads through it too	A same-stamp rewrite shows the new picture

Not covered by a test: the move and resize half of the modal-loop fix, which needs a real title-bar drag. It uses the same timer as the thumb.

Ten WindowsPixelTests baselines changed. The subtitle moved a few pixels, because its width is now the true advance, and in Selection the highlight now lines up exactly with its glyphs.

The full solution passes locally in Release: 2,078 passed, 24 skipped.