Adds two new config keys:
- paste_collapse_threshold (default: 5) — line count threshold for
bracketed paste collapse in both TUI and CLI
- paste_collapse_threshold_fallback (default: 0, disabled) — same for
the fallback heuristic in terminals without bracketed paste support
TUI frontend reads these from config.get full via applyDisplay/patchUiState.
CLI reads from self.config at paste-handling time.
Closes#5626
Related: #5623