Add pixel-based scroll distance support

[thymikee]

Summary

Add pixel-based scroll control with consistent cross-platform semantics.

Current behavior

scroll currently uses a relative amount on Android and ignores the amount on iOS.

Proposed scope

• Add scroll --pixels N.
• Define cross-platform behavior that maps to a predictable gesture distance.
• Keep existing directional scroll behavior backward-compatible.

Acceptance criteria

• Users can request a pixel-based scroll distance.
• Behavior is consistent enough across iOS and Android to use in agent flows.
• Docs explain the relationship between relative and pixel-based scroll modes.