feat: support SetHighlightedFunc for TextView region-based navigation

[cassdeckard]

Summary

Add support for SetHighlightedFunc on TextView so a callback runs when the highlighted region changes. This enables region-based navigation (e.g. switching pages when the user selects a different region).

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The presentation info bar uses a TextView with regions for each slide; when the highlighted region changes, the app calls SwitchToPage to show that slide.

Demo Usage

Info bar needs to call SwitchToPage(highlightedRegion) when region changes.

Proposed Approach

• Add onHighlighted (template expression) to TextView primitive config
• In builder/properties.go, call SetHighlightedFunc when onHighlighted is set
• Pass added/removed/remaining region IDs to the template (via state or function args)
• Consider related: feat: support SetInputCapture per primitive #13 (presentation navigation model)

Relevant Files

• config/types.go – add onHighlighted to TextView/Primitive
• builder/properties.go – applyTextViewProperties
• template/ – executor support for region args
• docs/tview-coverage.md – document once implemented