refactor(core): scaffold empty improvise-core sub-crate

Lay the groundwork for Phase B of improvise-36h: a new
`crates/improvise-core/` sub-crate that will house the pure-data core
(model, view, workbook, format) once the files move in the next commit.

Only scaffolding here:
- New `crates/improvise-core/Cargo.toml` mirroring improvise-formula's
  structure; declares deps on improvise-formula, anyhow, indexmap, serde.
- New `crates/improvise-core/src/lib.rs` with the single re-export
  `pub use improvise_formula as formula;` so that in the next commit the
  moved code's `crate::formula::*` paths resolve unchanged.
- Root `Cargo.toml` adds the new crate to the workspace members and the
  main crate's `[dependencies]`, ready to receive the move.

No source files change yet; cargo check --workspace still compiles as
before.

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

Cargo.lock

@@ -737,6 +737,7 @@ dependencies = [
  "dirs",
  "enum_dispatch",
  "flate2",
+ "improvise-core",
  "improvise-formula",
  "indexmap",
  "pest",
@@ -751,6 +752,17 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "improvise-core"
+version = "0.1.0-rc2"
+dependencies = [
+ "anyhow",
+ "improvise-formula",
+ "indexmap",
+ "proptest",
+ "serde",
+]
+
 [[package]]
 name = "improvise-formula"
 version = "0.1.0-rc2"

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = [".", "crates/improvise-formula"]
+members = [".", "crates/improvise-formula", "crates/improvise-core"]
 
 [package]
 name = "improvise"
@@ -18,6 +18,7 @@ name = "improvise"
 path = "src/main.rs"
 
 [dependencies]
+improvise-core = { path = "crates/improvise-core" }
 improvise-formula = { path = "crates/improvise-formula" }
 ratatui = "0.30"
 crossterm = "0.29"

crates/improvise-core/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "improvise-core"
+version = "0.1.0-rc2"
+edition = "2024"
+description = "Pure-data model, views, and workbook for improvise"
+license = "Apache-2.0"
+repository = "https://github.com/fiddlerwoaroof/improvise"
+
+[dependencies]
+improvise-formula = { path = "../improvise-formula" }
+anyhow = "1"
+indexmap = { version = "2", features = ["serde"] }
+serde = { version = "1", features = ["derive"] }
+
+[dev-dependencies]
+proptest = "1"

crates/improvise-core/src/lib.rs

@@ -0,0 +1,6 @@
+//! Pure-data core of the `improvise` project: `Model`, views, `Workbook`,
+//! and number formatting. Depends on `improvise-formula` for AST types;
+//! has no awareness of UI, I/O, or commands.
+//!
+//! Scaffolded empty in this commit; the modules land in the next commit.
+pub use improvise_formula as formula;