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"