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/src/lib.rs

@@ -0,0 +1,6 @@
+//! Pure-data core of the `improvise` project: `Model`, views, `Workbook`,
+//! and number formatting. Depends on `improvise-formula` for AST types;
+//! has no awareness of UI, I/O, or commands.
+//!
+//! Scaffolded empty in this commit; the modules land in the next commit.
+pub use improvise_formula as formula;