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.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"] }