rust: tui (canvas renderer, views, modals, crossterm event loop)

Port of internal/tui. Bubble Tea's message loop is replaced by a
crossterm input thread, timers and background commands feeding one
loop; the frame cache, canvas, colour quantisation, LOD treemap,
selection persistence and modal architecture are kept. Frames are
diffed against the previous one per line and per style run.
Exposes tui::bench::BenchModel and big_tree for benchmarks.

rust/src/lib.rs

@@ -12,6 +12,7 @@ pub mod scan;
 pub mod snapshot;
 pub mod textutil;
 pub mod treemap;
+pub mod tui;
 
 /// Boxed error used across mode boundaries.
 pub type Error = Box<dyn std::error::Error + Send + Sync>;