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>
