refactor: add Cmd trait with CmdContext and first command implementations

Define Cmd trait (execute returns Vec<Box<dyn Effect>>) and CmdContext
(read-only state snapshot). Implement navigation commands (MoveSelection,
JumpTo*, ScrollRows), mode commands (EnterMode, Quit, SaveAndQuit),
cell operations (ClearSelectedCell, YankCell, PasteCell), and view
commands (TransposeAxes, Save, EnterSearchMode).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/command/mod.rs

@@ -5,6 +5,7 @@
 //! The headless CLI (--cmd / --script) routes through here, and the TUI
 //! App also calls dispatch() for every user action that mutates state.
 
+pub mod cmd;
 pub mod dispatch;
 pub mod parse;
 pub mod types;