fix(sdl3): track mouse button state from events, not polling

[pdfinn]

SDL_GetMouseState() returns the instantaneous button state at the time
of the call, not the state when an event was queued. When a fast click
produces both BUTTON_DOWN and BUTTON_UP events before the event loop
polls them, SDL_GetMouseState() during BUTTON_DOWN processing already
shows the button released — the entire click is silently lost.

This is the root cause of middle-click on Xenith tags intermittently
failing to execute: middle-clicks tend to be quick taps, so both events
queue before the poll loop runs, and the button-down is reported as
buttons=0. Left-clicking first appeared to "fix" it because the user
would then click more deliberately, giving the event loop time to
process the down event before the up arrived.

Replace SDL_GetMouseState() polling with event-based button tracking:
BUTTON_DOWN sets the bit, BUTTON_UP clears it. This mirrors how the
X11 backend (win-x11a.c) derives state from the event structure.

Also fixes: the sdl3_mainloop event handler used event.button.x/y for
MOUSE_MOTION events, reading from the wrong union member. Now motion
events correctly use event.motion.x/y.

https://claude.ai/code/session_01W4LrxzkC8jGBN1xi177Wvg