Scroll twice as far, with a setting for how far - #26

Closes #24. The trackpad and the mouse wheel now scroll twice as far as before, and Settings › Shell › Scrolling sets how far: from 1×, SwiftTerm's own rate, to 5×, in half steps.

Why it was slow

SwiftTerm moves the scrollback with the fingers, pixel for pixel (trackpad deltas divided by the row height, the remainder kept for the next event), and a notch of the mouse wheel moves one line. That feels natural over a page but takes many swipes through a long output. SwiftTerm already scales the wheel by scrollSensitivity; nothing set it.

Summary

Preferences.scrollSpeed        1…5, 2 by default; a stored value out of range falls back to 2
  → Pane.applyPreferences      terminal.scrollSensitivity = scrollSpeed (every pane, as soon as it changes)
  → SwiftTerm scrollWheel      delta × sensitivity, then: scrollback, mouse reports, or arrow keys

• It applies in full-screen programs too (vim, less), since SwiftTerm scales the delta before deciding where it goes. SwiftTerm still caps how many reports or keys one event sends.
• Settings › Shell gets a Scrolling section with a slider, next to Selection. No screenshot shows that page, so none were retaken.
• Docs: settings.md and command-line.md.

Tests

• PaneTests.aWheelNotchScrollsAsFarAsTheSettingSays sends real line-unit wheel events to a pane in a window: a notch moves 2 lines by default, 4 at 4×.
• PreferencesTests cover the default, keeping a change, and repairing a value out of range.
• All three CI gates pass locally: build with tests, the full suite (217 + 350), and the universal release app.

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