feat(tui): replace O(N) scanning with progressive loading and VisualHeightIndex

Replace synchronous file loading with AppLoadingState state machine (Empty/Loading/Ready/Error) for instant interactivity. Add ViewportCache for on-demand viewport computation, replacing global wrap/level caches. Integrate background indexer polling and file watcher events into the TUI event loop. Add loading UI with progress percentage, estimated line numbers with ~ prefix, and error state display. Eliminate all O(N) linear scans using VisualHeightIndex binary search.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/tui/Cargo.toml

@@ -10,6 +10,7 @@ clap.workspace = true
 anyhow.workspace = true
 log-viewer-core.workspace = true
 serde_json.workspace = true
+crossbeam-channel.workspace = true
 
 [dev-dependencies]
 tempfile = { workspace = true }