feat: Forth-style prefix command parser

Replace JSON command syntax with prefix notation: `word arg1 arg2`.
Multiple commands per line separated by `.`. Coordinate pairs use
`Category/Item`. Quoted strings for multi-word values. set-cell
uses value-first: `set-cell 100 Region/East Measure/Revenue`.

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

src/command/mod.rs

@@ -6,7 +6,9 @@
 //! App also calls dispatch() for every user action that mutates state.
 
 pub mod dispatch;
+pub mod parse;
 pub mod types;
 
 pub use dispatch::dispatch;
+pub use parse::parse_line;
 pub use types::{Command, CommandResult};