feat: expose TextView SetScrollable and SetChangedFunc in YAML

[cassdeckard]

Summary

Expose TextView's SetScrollable and SetChangedFunc in YAML config so views can be non-scrollable and trigger redraws or callbacks when content changes (e.g. for live-updating displays).

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The TextView1 slide uses SetScrollable(false) and SetChangedFunc for a live-updating counter that writes to the TextView from a goroutine.

Demo Usage

TextView1 uses SetScrollable(false) and SetChangedFunc for live updates.

Proposed Approach

• Add scrollable: false (default true) and onChanged (template expression) to TextView primitive
• In builder/properties.go, call SetScrollable and SetChangedFunc when configured
• For onChanged, the callback can trigger app.Draw() or run a template action

Relevant Files

• config/types.go – add scrollable, onChanged for textView
• builder/properties.go – applyTextViewProperties
• docs/tview-coverage.md – document once implemented