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>

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = [".", "crates/improvise-formula"]
+members = [".", "crates/improvise-formula", "crates/improvise-core"]
 
 [package]
 name = "improvise"
@@ -18,6 +18,7 @@ name = "improvise"
 path = "src/main.rs"
 
 [dependencies]
+improvise-core = { path = "crates/improvise-core" }
 improvise-formula = { path = "crates/improvise-formula" }
 ratatui = "0.30"
 crossterm = "0.29"