docs(test-evidence): what this wave's persistence and view tests are evidence of

Keyed by test name per design-principles section 7, recording for each one the
bug it reproduced, the fault injected to watch it fail, and -- where a test has
a neighbour that looks like it -- the input that separates the two.

Three entries are about that separation rather than about a bug:

- a_view_that_does_not_prune_writes_no_prune_line is the direction the default
  supplies, so a writer emitting "prune: true" unconditionally passes the real
  test and fails this one.
- an_interleaved_category_keeps_its_item_order explains why the ticket's own
  two-item reproduction cannot discriminate: with one item of each kind, both
  the right and the wrong order are a single bare run followed by a single
  grouped one.
- slash_in_hidden_name_survives_roundtrip records that improvise-nvq4 does NOT
  reproduce, and is kept as a passing control so the claim cannot be re-filed
  without someone first watching it pass.

core.md is touched for the improvise-qx1z entries, which are View-level and
belong there rather than in formula-and-io.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

context/test-evidence/core.md

@@ -351,6 +351,41 @@ test flips the category back to `Page` afterwards, so it distinguishes "omits
 The fixture puts a category on each of `Page`, `Filter`, `Row` and `None` so the
 result separates the two questions rather than merely counting.
 
+### on_category_removed_forgets_a_collapsed_group
+
+improvise-qx1z arm 2. `View::on_category_removed` cleared `category_axes`,
+`page_selections` and `hidden_items` — but not `collapsed_groups`. The other
+three lines being right there is what makes this an omission rather than a
+decision: collapse `Month/Q1`, delete the category, re-add it with `Jan[Q1]`,
+and the group comes back **pre-collapsed** with nothing saying why. Reachable
+from shipped keys, through the category panel.
+
+Fault injected: delete the added line; the test fails at
+`is_group_collapsed("Month", "Q1")`. The test re-adds the category after
+removing it, because a view that merely *forgot the axis* would also report
+`false` for a category it no longer knows — asking the question after the
+re-add is what separates "cleared" from "not currently registered".
+
+### on_item_removed_forgets_a_hidden_item
+
+improvise-qx1z arm 1, at the `View` level. `Model::remove_item` removes the item
+and its cells and **touches no view at all**, so `hidden_items["Region"]` kept
+`"East"`: re-adding the item put it in the model and in the category panel while
+leaving it absent from the grid, with no indication why.
+
+The fixture hides **two** items and removes one. Without the second, a fix that
+dropped the whole category entry — `hidden_items.remove(cat_name)` — would pass
+while destroying unrelated state; the surviving `West` assertion is the input
+that separates "forget this item" from "forget this category".
+
+### on_item_removed_forgets_a_page_selection_naming_it
+
+The third question improvise-qx1z asked to check while there, and the answer was
+the same: a page selection naming a removed item survived it. Asserted in both
+directions in one test — removing a *different* item must leave the selection
+alone — because a hook that cleared the selection unconditionally would satisfy
+the removal half and silently reset the user's page on every unrelated delete.
+
 ## `view/layout.rs`
 
 ### prune_axis_items_indexes_data_positions_not_visual_ones