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

Lay groundwork for Phase 3 of the workspace split (improvise-8zh): a new
`crates/improvise-io/` sub-crate that will house the persistence and
import layers once the files move in the next commit.

Only scaffolding here:
- New `crates/improvise-io/Cargo.toml` declares deps on improvise-core,
  improvise-formula, and the external crates persistence+import already
  use: anyhow, chrono, csv, flate2, indexmap, pest, pest_derive, serde,
  serde_json. Dev-deps: pest_meta, proptest, tempfile.
- New `crates/improvise-io/src/lib.rs` re-exports the core modules under
  their conventional names (`format`, `model`, `view`, `workbook`,
  `formula`) so in the next commit the moved code's `crate::model::*`,
  `crate::view::*`, `crate::workbook::*`, `crate::format::*`, and
  `crate::formula::*` paths resolve unchanged.
- Root `Cargo.toml` adds the new crate to 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

@@ -739,6 +739,7 @@ dependencies = [
  "flate2",
  "improvise-core",
  "improvise-formula",
+ "improvise-io",
  "indexmap",
  "pest",
  "pest_derive",
@@ -775,6 +776,26 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "improvise-io"
+version = "0.1.0-rc2"
+dependencies = [
+ "anyhow",
+ "chrono",
+ "csv",
+ "flate2",
+ "improvise-core",
+ "improvise-formula",
+ "indexmap",
+ "pest",
+ "pest_derive",
+ "pest_meta",
+ "proptest",
+ "serde",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.14.0"

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = [".", "crates/improvise-formula", "crates/improvise-core"]
+members = [".", "crates/improvise-formula", "crates/improvise-core", "crates/improvise-io"]
 
 [package]
 name = "improvise"
@@ -20,6 +20,7 @@ path = "src/main.rs"
 [dependencies]
 improvise-core = { path = "crates/improvise-core" }
 improvise-formula = { path = "crates/improvise-formula" }
+improvise-io = { path = "crates/improvise-io" }
 ratatui = "0.30"
 crossterm = "0.29"
 serde = { version = "1", features = ["derive"] }

crates/improvise-io/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "improvise-io"
+version = "0.1.0-rc2"
+edition = "2024"
+description = "Persistence and import for improvise (.improv format, CSV, JSON wizard)"
+license = "Apache-2.0"
+repository = "https://github.com/fiddlerwoaroof/improvise"
+
+[dependencies]
+improvise-core = { path = "../improvise-core" }
+improvise-formula = { path = "../improvise-formula" }
+anyhow = "1"
+chrono = { version = "0.4", features = ["serde"] }
+csv = "1"
+flate2 = "1"
+indexmap = { version = "2", features = ["serde"] }
+pest = "2.8.6"
+pest_derive = "2.8.6"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[dev-dependencies]
+pest_meta = "2.8.6"
+proptest = "1"
+tempfile = "3"

crates/improvise-io/src/lib.rs

@@ -0,0 +1,15 @@
+//! I/O layer for `improvise`: `.improv` persistence (parse/format, save/load,
+//! CSV export) and import (CSV/JSON wizard, field analyzer).
+//!
+//! Depends on `improvise-core` for the data model (`Model`, `View`,
+//! `Workbook`, `CellKey`, `CellValue`, `GridLayout`, `Axis`, `Group`) and on
+//! `improvise-formula` for formula parsing. Has no awareness of UI or
+//! commands — builds standalone via `cargo build -p improvise-io`.
+//!
+//! Re-exports the core modules under their conventional names so code in
+//! this crate can keep using `crate::model::*`, `crate::view::*`,
+//! `crate::workbook::*`, `crate::format::*`, and `crate::formula::*` paths.
+//!
+//! Scaffolded empty in this commit; the modules land in the next commit.
+pub use improvise_core::{format, model, view, workbook};
+pub use improvise_formula as formula;