Merge pull request #241 from fiddlerwoaroof/fix/djcn-canonical-debug

Make Model's Debug exhaustive and the Debug fingerprint canonical across instances (improvise-djcn)

context/test-evidence/command.md

@@ -205,6 +205,28 @@ Row, swapping the axes and rotating Row→Column land on the same view, and an
 equivalence test that cannot distinguish the wrong effect from the right one
 proves nothing (`doctrine(check_the_checker)`).
 
+**The fixture needs cells and multi-entry view maps too (improvise-djcn).** The
+test compares `format!("{:?}", workbook)` across **two** `App`s. It passed only
+because the fixture had no cells and every view map had at most one entry, and a
+`HashMap` of one entry has only one order. The fixture now carries:
+- three cells;
+- two page selections;
+- two categories with hidden items, one of them with two;
+- two categories with collapsed groups.
+
+That fixture was red in 3 of 3 runs on the old `Debug` impls: the by-key and
+by-command fingerprints differed only in hash order. It was green after the fix.
+
+Each half of the fix was checked by restoring the derived `Debug` on its own:
+- `View`'s derived `Debug` alone: 3 of 3 runs red.
+- `DataStore` and `SymbolTable`'s derived `Debug` alone: 3 of 3 runs red.
+
+So neither half is carried by the other. The exhaustive destructure in `Model`'s
+`Debug` has no test of its own, because it is a compile-time guard. It was
+checked by adding a `probe_field` to `Model` and to `Model::new`. The build then
+failed with `E0027: pattern does not mention field probe_field` at the `Debug`
+impl, and nowhere else.
+
 ### a_stale_wire_model_command_cannot_be_applied
 
 A wire `Command::Model` cannot reach the document without its basis being checked