Create the mpv host window with WS_DISABLED so Windows skips the video subtree
when it picks the window that owns a contact and hands the input to the parent
Flutter view instead. Touch over the video never reached Flutter before: mpv's
inner window owns the contact from its own thread, and neither relay worked
from there - Flutter resolves WM_POINTER with GetPointerInfo, which only
answers for a message the calling thread retrieved, and the system discards a
cross-thread pointer send outright. WS_EX_TRANSPARENT and an HTTRANSPARENT
WM_NCHITTEST reply are both same-thread-only, so disabling the subtree is the
one hit-test opt-out that applies across threads. The mouse relay stays for
input that still reaches mpv's window.
Repair the contract test that covers this. It drove its pointer assertions with
cross-thread sends that Windows drops, so every touch assertion had been dead
since it was added and the suite fails "primary touch must press once" on main.
Relaying those sends through the window's own thread runs all eight tests, and
injected mouse and touch presses over the disabled host now assert delivery to
the parent view; removing WS_DISABLED fails the suite.
close#1556
edde746