feat: make TextView region count configurable (Tab/Backtab)

[cassdeckard]

Summary

Make the TextView region Tab/Backtab navigation configurable. Currently builder/properties.go hardcodes a cycle of 3 regions; the presentation demo uses 9 regions. The count should be configurable or derived from the region tags in the text.

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The TextView2 slide uses 9 regions (["0"] through ["8"]); Tab/Backtab should cycle through all of them.

Demo Usage

Tab/Backtab in builder/properties.go hardcoded to 3; demo uses 9.

Proposed Approach

• Add regionCount: N to TextView config; if unset, derive from region tags in text (parse ["X"] and take max+1)
• Or: always derive dynamically from GetHighlights and available region IDs at runtime
• Update applyTextViewProperties region navigation logic to use configurable count

Relevant Files

• config/types.go – add regionCount (optional)
• builder/properties.go – applyTextViewProperties region handler
• docs/tview-coverage.md – document once implemented