Merge pull request #56 from fiddlerwoaroof/docs/record-cell-key-reason

docs(core): correct the recorded reason for leaving record_cell_key's Category arm untested

context/code/core.md

@@ -434,6 +434,36 @@ Registration is guaranteed for every model category: `Workbook::new`,
 `cycle_axis` does, and no-ops on an unknown category to match `set_axis`
 (improvise-t8s).
 
+### `record_cell_key` is a match, and the match is the guard (improvise-ir7l)
+
+`GridLayout::record_cell_key(row, col)` is the one mapping from a records
+position to a model cell, and it answers `Some` for `RecordCol::Measure` alone.
+The other three arms are spelled out rather than swept into a `_`, and that
+exhaustive match is the whole guard: a new `RecordCol` variant fails the build
+until someone decides whether it names a stored cell.
+
+**There is no test for the `Category` arm, and the reason is not that the wrong
+answer would be harmless.** It would be a real fault, and a subtle one worth
+naming so nobody re-derives it wrongly. `RecordCol::Category(name)` carries a
+**category** name, while `RecordRow::cell_key(arg)` puts its argument under
+`_Measure`. So answering `Some` there yields `coords + _Measure=<category
+name>`, and `Model::formula_for` would match it whenever a `_Measure` item
+happens to share a name with a regular category — projecting `CellId::Derived`
+for a **coordinate column**, which is never a formula target and which the
+pivot path does not have at all. The two front ends would then disagree, which
+is the design-principles §5 defect this ticket exists to close. `RecordCol`
+exists for exactly that collision: its doc says the enum resolves a column from
+its index "so a category and a measure sharing a name stay distinguishable".
+
+What earns the decision to leave it untested is narrower: the **reachable**
+version of that fault is a new variant answered carelessly, and the exhaustive
+match makes that unrepresentable rather than merely unlikely
+(`doctrine(an_invariant_is_made_unrepresentable_not_validated)`). A test would
+pin behaviour the type system already pins, and would not survive the arm being
+renamed. Found in review, and the first recorded reason for declining it was
+**wrong** — it claimed the injected behaviour was defensible by analogy with a
+formula targeting a regular category, which is a different key entirely.
+
 ## `PageAxis` — what constrains, and what the user may turn (improvise-p1ym)
 
 `GridLayout::page_coords` answers two questions that gotcha 27 says must not be