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>

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"