refactor(formula): extract formula parser into separate crate

Extract the formula AST and parser into a dedicated `improvise-formula`
crate and convert the project into a Cargo workspace.

The root crate now re-exports `improvise-formula` as `crate::formula` to
maintain backward compatibility for internal callers. The repository map is
updated to reflect the new crate structure.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

src/lib.rs

@@ -1,7 +1,7 @@
 pub mod command;
 pub mod draw;
 pub mod format;
-pub mod formula;
+pub use improvise_formula as formula;
 pub mod import;
 pub mod model;
 pub mod persistence;