Fix spurious "MPV start timed out" on Windows (named-pipe readiness check)

[cognis-digital]

Problem

On Windows, MPV(start_mpv=True, ...) reliably fails with MPVError: MPV start timed out even when MPV launches correctly and the IPC pipe is live.

Root cause

MPVProcess.__init__ polls for the IPC endpoint with:

for _ in range(100):
    time.sleep(0.1)
    if os.path.exists(ipc_socket):   # ipc_socket == r'\.\pipe\<name>' on Windows
        ...

os.path.exists() is always False for Windows named-pipe paths (\.\pipe\...) — os.stat can't stat a pipe — so the loop never detects the pipe and the else branch raises MPV start timed out, despite MPV being up. POSIX is unaffected because there the endpoint really is a filesystem socket.

Fix

Add _ipc_endpoint_ready(): keep os.path.exists on POSIX, but on Windows probe the pipe with _winapi.WaitNamedPipe (the transport layer already depends on _winapi). Minimal, stdlib-only, no new deps, no API change.

Verification

Against MPV v0.41 on Windows 10 / Python 3.14:

• Before: MPVError: MPV start timed out (every run)
• After: MPV connects in ~0.5s; idle_active and subsequent commands work.

m = MPV(start_mpv=True, ipc_socket='test', mpv_location=r'...\mpv.exe', vid='no')
# before: raises MPV start timed out
# after:  connects, m.idle_active -> True