Ignore transient opposite scroll pulses before reversing

[seng1e]

Summary

Improve scroll smoothing stability by requiring confirmation before treating a brief opposite-direction pulse as a real reversal.

Related issue

• likely addresses scroll bounce back #568 (scroll bounce back), where users report one-direction scrolling occasionally reversing or bouncing back

What changed

• add per-axis reversal candidate tracking in ScrollPoster
• ignore the first opposite pulse while residual scroll is still active
• accept reversal on the next matching opposite pulse and reset the smoothing filter
• add focused tests for continuation, ignored opposite pulse, confirmed reversal, and immediate reversal without residual momentum

Root cause

Mos previously reset scroll direction immediately when any opposite-direction input arrived. On some mice, a transient opposite pulse can appear during fast wheel motion. That single pulse was enough to reset the scroll buffer and produce a visible reverse spike.

Validation

• built patched release Mos.app locally with Xcode 26.4
• installed the patched app over the local Mos.app and verified it launches
• manual testing on the target machine no longer reproduces the random reverse-scroll spike

Notes

This keeps intentional direction changes working, but adds a small confirmation guard while residual smooth-scroll output is still active.

[seng1e]

Related issue: #568 (scroll bounce back). The symptoms described there match this fix very closely: during one-direction wheel scrolling, the output can briefly reverse or "bounce back". That issue also has multiple confirmations across different macOS versions and mice, which lines up with the transient opposite-pulse behavior addressed by this PR.