feat(core): raise MAX_CATEGORIES from 12 to 24, with one home (improvise-felo.1)

The wizard's private copy of the limit is gone; it reads the model's
now-public constant. Tests that fill to the cap derive it. Why 12 was
chosen and what 24 costs: context/code/core.md section 9.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/design-principles.md

@@ -69,7 +69,7 @@ same effect without the early return.
   three shapes; dispatch pattern-matches exhaustively.
 
 - `CategoryKind` is `Regular | VirtualIndex | VirtualDim | VirtualMeasure | Label`.
-  Business rules (e.g., the 12-category limit counts only `Regular`) are
+  Business rules (e.g., the 24-category limit counts only `Regular`) are
   enforced by matching on the enum, not by checking name prefixes. Virtual
   categories (`_Index`, `_Dim`, `_Measure`) always exist: `_Index` and `_Dim`
   support drill-down/records mode; `_Measure` holds numeric data fields and
@@ -99,7 +99,8 @@ sort. Property tests verify this invariant.
 - `CellKey::new()` is the only way to build a key — it always sorts.
 - `Category::add_item()` deduplicates by name and auto-assigns IDs via a private
   counter. External code cannot fabricate an `ItemId`.
-- `Model::add_category()` checks the 12-category limit before insertion.
+- `Model::add_category()` checks the category limit (`MAX_CATEGORIES`, 24)
+  before insertion.
 - `Formula::new()` takes all required fields; there is no default/empty formula
   to accidentally leave half-initialized.