Fix trackpad fractional-scroll loss and first-click focus in splits

[sbertix]

Closes #736

Summary

Two independent input defects from the issue:

• Precision scroll dropped its fractional remainder. Surface.scrollCallback
  stored poff - amount * cell_size as the pending scroll, but
  amount = poff / cell_size, so that expression equals poff and the sub-row
  remainder was zeroed on every row emitted. Slow trackpad scrolls lost motion
  (with a 16px cell and repeated 7px deltas, rows landed on events 3/6/9 instead
  of 3/5/7). Fixed by truncating to whole rows first and carrying
  poff - trunc(amount) * cell_size, on both axes. Shipped as an out-of-tree
  patch applied at build time; the ghostty submodule pin is untouched.
• First click on an unfocused split misbehaved. A click on an unfocused split
  while the app and window are already active only transfers focus, but the click
  was forwarded to the terminal, leaking a press (and later an orphaned release)
  that broke the first drag-select. The click is now consumed for focus transfer,
  and every terminal release is gated on a press this view actually sent, so
  neither a same-split click nor a drag that presses in one split and releases in
  another orphans a release. Hit-testing now happens in content-view space so the
  check holds when the split is scrolled into scrollback, focus is gated on the
  window's first responder rather than a cached bit, and Force Touch pressure is
  reset on the swallowed release.

Type of change

• Bug fix (the linked issue is a bug report)
• Feature (the linked issue is a feature request marked ready)
• Documentation
• Other (please describe)

How was this tested?

• make check passes (format + lint)
• make test passes
• I built and ran the app to confirm the change works

Checklist

• This pull request is linked to an issue with Closes # above.
• For a feature, the linked issue is labeled ready.
• I am the author of this work and accountable for it; no commit is authored or co-authored by an AI agent.
• I have read the Contributing guide and the Code of Conduct.