feat(state): add file‑tree and repository‑search state modules

Introduce `FileTreeState` for managing a navigable file hierarchy with Git decorations, filtering, and cursor/scroll handling.
Add `RepoSearchState` and related types to support asynchronous ripgrep‑backed repository searches, including result aggregation, pagination, and UI interaction.

crates/owlen-tui/src/state/mod.rs

@@ -6,5 +6,20 @@
 //! to test in isolation.
 
 mod command_palette;
+mod file_tree;
+mod search;
+mod workspace;
 
 pub use command_palette::{CommandPalette, ModelPaletteEntry};
+pub use file_tree::{
+    FileNode, FileTreeState, FilterMode as FileFilterMode, GitDecoration, VisibleFileEntry,
+};
+pub use search::{
+    RepoSearchFile, RepoSearchMatch, RepoSearchMessage, RepoSearchRow, RepoSearchRowKind,
+    RepoSearchState, SymbolEntry, SymbolKind, SymbolSearchMessage, SymbolSearchState,
+    spawn_repo_search_task, spawn_symbol_search_task,
+};
+pub use workspace::{
+    CodePane, CodeWorkspace, EditorTab, LayoutNode, PaneDirection, PaneId, PaneRestoreRequest,
+    SplitAxis, WorkspaceSnapshot,
+};