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)

Cargo.lock

@@ -737,6 +737,7 @@ dependencies = [
  "dirs",
  "enum_dispatch",
  "flate2",
+ "improvise-formula",
  "indexmap",
  "pest",
  "pest_derive",
@@ -750,6 +751,14 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "improvise-formula"
+version = "0.1.0-rc2"
+dependencies = [
+ "anyhow",
+ "serde",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.14.0"

Cargo.toml

@@ -1,3 +1,6 @@
+[workspace]
+members = [".", "crates/improvise-formula"]
+
 [package]
 name = "improvise"
 version = "0.1.0-rc2"
@@ -15,6 +18,7 @@ name = "improvise"
 path = "src/main.rs"
 
 [dependencies]
+improvise-formula = { path = "crates/improvise-formula" }
 ratatui = "0.30"
 crossterm = "0.29"
 serde = { version = "1", features = ["derive"] }

context/repo-map.md

@@ -1,8 +1,10 @@
 # Repository Map (LLM Reference)
 
 Terminal pivot-table modeling app. Rust, Ratatui TUI, command/effect architecture.
-Crate `improvise` v0.1.0-rc2, Apache-2.0, edition 2024.
+Cargo workspace, Apache-2.0, edition 2024. Root package `improvise` v0.1.0-rc2.
 Library + binary crate: `src/lib.rs` exports public modules, `src/main.rs` is the CLI entry.
+Sub-crates live under `crates/`:
+- `crates/improvise-formula/` — formula parser, AST (`Expr`, `BinOp`, `AggFunc`, `Formula`, `Filter`), `parse_formula`. Re-exported as `crate::formula` from the main crate via `pub use improvise_formula as formula;`.
 
 ---
 
@@ -153,7 +155,7 @@ pub enum CategoryKind { Regular, VirtualIndex, VirtualDim, VirtualMeasure, Label
 ### Formula System
 
 ```rust
-// src/formula/ast.rs
+// crates/improvise-formula/src/ast.rs
 pub enum Expr {
     Number(f64),
     Ref(String),                           // reference to an item name
@@ -172,7 +174,7 @@ pub struct Formula {
     pub filter: Option<Filter>,   // WHERE clause
 }
 
-// src/formula/parser.rs
+// crates/improvise-formula/src/parser.rs
 pub fn parse_formula(raw: &str, target_category: &str) -> Result<Formula>
 ```
 
@@ -383,11 +385,11 @@ Lines / tests / path — grouped by layer.
    6 /  0t  model/mod.rs
 ```
 
-### Formula layer
+### Formula layer (sub-crate `improvise-formula` under `crates/`)
 ```
- 461 / 29t  formula/parser.rs     Recursive descent parser → Formula AST
-  77 /  0t  formula/ast.rs        Expr, BinOp, AggFunc, Formula, Filter (data only)
-   5 /  0t  formula/mod.rs
+ 776 / 35t  crates/improvise-formula/src/parser.rs  Recursive descent parser → Formula AST
+  77 /  0t  crates/improvise-formula/src/ast.rs     Expr, BinOp, AggFunc, Formula, Filter (data only)
+   5 /  0t  crates/improvise-formula/src/lib.rs
 ```
 
 ### View layer

crates/improvise-formula/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "improvise-formula"
+version = "0.1.0-rc2"
+edition = "2024"
+description = "Formula parser and AST for improvise"
+license = "Apache-2.0"
+repository = "https://github.com/fiddlerwoaroof/improvise"
+
+[dependencies]
+anyhow = "1"
+serde = { version = "1", features = ["derive"] }

crates/improvise-formula/src/parser.rs

@@ -460,7 +460,7 @@ fn parse_comparison(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
 #[cfg(test)]
 mod tests {
     use super::parse_formula;
-    use crate::formula::{AggFunc, BinOp, Expr};
+    use crate::{AggFunc, BinOp, Expr};
 
     #[test]
     fn parse_simple_subtraction() {

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;