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>

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"