v0.4.0

Added

• Fall back to plain-text viewing when the interactive viewer auto-detects a
  structured file type but cannot format the input, with a temporary red footer
  notice that explains the fallback and points users to --type for an
  explicit override.

Changed

• Reorganize format, load, diff, TUI, and viewer module boundaries so future
  format support can share the same lazy loading, rendering, navigation, and
  highlighting architecture more cleanly.