docs(core): note improvise-core layout in repo-map

Reflect Phase B of improvise-36h: `model/`, `view/`, `workbook.rs`, and
`format.rs` now live in the `improvise-core` sub-crate under `crates/`.

- Sub-crate list expanded to describe improvise-core's scope, its
  dependency on improvise-formula, and the standalone-build guarantee.
- File Inventory reorganized into a single "Core crate layers" block
  covering model/view/workbook/format, with paths rooted at
  `crates/improvise-core/src/`.
- Updated line/test counts to match current contents (Phase A + merge
  with main brought in records mode, IndexMap-backed DataStore, etc).

No architectural change; the main crate's re-exports keep every
`crate::model`/`crate::view`/`crate::workbook`/`crate::format` path
resolving unchanged, so no "How to Find Things" table edits are needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

context/repo-map.md

@@ -2,8 +2,9 @@
 
 Terminal pivot-table modeling app. Rust, Ratatui TUI, command/effect architecture.
 Cargo workspace, Apache-2.0, edition 2024. Root package `improvise` v0.1.0-rc2.
-Library + binary crate: `src/lib.rs` exports public modules, `src/main.rs` is the CLI entry.
+Library + binary crate: `src/lib.rs` re-exports public modules (many from sub-crates), `src/main.rs` is the CLI entry.
 Sub-crates live under `crates/`:
+- `crates/improvise-core/` — pure-data core: `Model`, `View`, `Workbook`, and number formatting. Depends on `improvise-formula`. Re-exported from the main crate so `crate::model`, `crate::view`, `crate::workbook`, `crate::format` still resolve everywhere. Has no awareness of UI, I/O, or commands — builds standalone via `cargo build -p improvise-core`.
 - `crates/improvise-formula/` — formula parser, AST (`Expr`, `BinOp`, `AggFunc`, `Formula`, `Filter`), `parse_formula`. Re-exported as `crate::formula` from the main crate via `pub use improvise_formula as formula;`.
 
 ---
@@ -376,30 +377,44 @@ Import flags: `--category`, `--measure`, `--time`, `--skip`, `--extract`, `--axi
 
 Lines / tests / path — grouped by layer.
 
-### Model layer
+### Core crate layers (sub-crate `improvise-core` under `crates/`)
+All of `model/`, `view/`, `workbook.rs`, `format.rs` now live under
+`crates/improvise-core/src/`. Paths below are relative to that root.
+The main crate re-exports each as `crate::model`, `crate::view`,
+`crate::workbook`, `crate::format` via `src/lib.rs`, so every
+consumer path stays unchanged.
+
+#### Model layer
 ```
-1692 / 66t  model/types.rs        Model struct, formula eval, CRUD, MAX_CATEGORIES=12
- 621 / 28t  model/cell.rs         CellKey (canonical sort), CellValue, DataStore (interned)
- 216 /  6t  model/category.rs     Category, Item, Group, CategoryKind
+2062 / 70t  model/types.rs        Model struct, formula eval, CRUD, MAX_CATEGORIES=12
+ 650 / 28t  model/cell.rs         CellKey (canonical sort), CellValue, DataStore (interned, sort_by_key)
+ 222 /  6t  model/category.rs     Category, Item, Group, CategoryKind
   79 /  3t  model/symbol.rs       Symbol interning (SymbolTable)
    6 /  0t  model/mod.rs
 ```
 
-### Formula layer (sub-crate `improvise-formula` under `crates/`)
+#### View layer
 ```
- 776 / 35t  crates/improvise-formula/src/parser.rs  Recursive descent parser → Formula AST
-  77 /  0t  crates/improvise-formula/src/ast.rs     Expr, BinOp, AggFunc, Formula, Filter (data only)
-   5 /  0t  crates/improvise-formula/src/lib.rs
-```
-
-### View layer
-```
-1013 / 23t  view/layout.rs        GridLayout (pure fn of Model+View), records mode, drill
- 521 / 28t  view/types.rs         View config (axes, pages, hidden, collapsed, format)
+1140 / 24t  view/layout.rs        GridLayout (pure fn of Model+View), records mode, drill
+ 531 / 28t  view/types.rs         View config (axes, pages, hidden, collapsed, format), none_cats()
   21 /  0t  view/axis.rs          Axis enum {Row, Column, Page, None}
    7 /  0t  view/mod.rs
 ```
 
+#### Workbook + format (top-level in improvise-core)
+```
+ 259 / 11t  workbook.rs           Workbook wraps Model + views; cross-slice category/view ops
+ 229 / 29t  format.rs             format_f64, parse_number_format (display-only rounding)
+  12 /  0t  lib.rs                Module declarations + `pub use improvise_formula as formula;`
+```
+
+### Formula layer (sub-crate `improvise-formula` under `crates/`)
+```
+ 776 / 65t  crates/improvise-formula/src/parser.rs  PEG parser (pest) → Formula AST
+  77 /  0t  crates/improvise-formula/src/ast.rs     Expr, BinOp, AggFunc, Formula, Filter (data only)
+   5 /  0t  crates/improvise-formula/src/lib.rs
+```
+
 ### Command layer
 ```
 command/cmd/                       Cmd trait, CmdContext, CmdRegistry, 40+ commands