Add an accessible screen-reader rendering mode

[AlexSkrypnyk]

Summary

Add an accessible rendering mode that presents a form as a linear sequence of plain text prompts for screen-reader and assistive-technology users, instead of the full-screen panel repaint.

Details

The interactive panel TUI redraws the whole viewport on every change. Full-screen repaint is hostile to screen readers, which re-read the entire screen on each repaint, so assistive-technology users cannot practically complete a panel form today.

Add an accessible mode that renders the same form as a linear, one-question-at-a-time sequence of plain, self-contained text prompts, each echoed as ordinary output lines. It should reuse the existing validation, derived-value, conditional-field, fix-up and provenance logic so only the presentation layer differs: a form defined once is answerable in either mode with no separate definition.

Enable it explicitly with a form-level option and implicitly via an environment variable, alongside the existing Unicode/ASCII and NO_COLOR display-mode switches.

Acceptance criteria

• A form-level option (and an environment variable) selects accessible mode.
• In accessible mode each field is presented as a discrete, linear prompt with no full-screen repaint or cursor addressing.
• Validation, required, default, derived, conditional and fix-up behaviour match the interactive mode; the resulting Answers (values and provenance) are identical for identical input.
• Every widget type degrades to a sensible linear prompt in accessible mode.
• Behaviour is covered by unit tests, including provenance parity with interactive collection.
• The mode is documented in the README display-modes section.