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.lock

@@ -737,6 +737,7 @@ dependencies = [
  "dirs",
  "enum_dispatch",
  "flate2",
+ "improvise-core",
  "improvise-formula",
  "indexmap",
  "pest",
@@ -751,6 +752,17 @@ dependencies = [
  "unicode-width",
 ]
 
+[[package]]
+name = "improvise-core"
+version = "0.1.0-rc2"
+dependencies = [
+ "anyhow",
+ "improvise-formula",
+ "indexmap",
+ "proptest",
+ "serde",
+]
+
 [[package]]
 name = "improvise-formula"
 version = "0.1.0-rc2"