improvise-yb4b/b5dp: pin the scan against the per-member fold; record the ruling, the bench and the evidence

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

context/test-evidence/command.md

@@ -6118,3 +6118,26 @@ four sites. It reverts each `effect::refuse(` to `effect::set_status(` alone and
 runs `cargo test --workspace --no-fail-fast`. Four of four fail this test, and
 nothing else fails. None of the four needs an abort: each is the only effect its
 command emits, and `p`/`i` are single bindings, not sequences.
+
+### a_formula_rows_error_does_not_poison_the_pooled_total
+
+improvise-b5dp, the user's ruling on the conflict between wm20's rulings (1) and
+(2): a formula column's total is the **pooled ratio**. A row's formula error
+does not poison it, and a stored error still does
+(`an_error_in_any_row_makes_the_total_an_error`). Rows North 25/30, South 27/30
+and East -4/0 draw 0.83, 0.90 and `div/0`. The total must read **0.80**
+(48/60), not an error. The test asserts that East's row really draws the error,
+so a fixture where no row errors cannot pass it.
+
+It was **green at `9948aaf5`**, because #239 already implemented the choice the
+user then ratified. It is a pin, not a reproduction. Rival: in `grid_body`'s
+walk, a drawn cell's `Error` overwrote its column's total. That is ruling (2)
+applied to every row. This test and its Share sibling fail on the total
+assertion. The other seven `column_totals` tests pass.
+
+### a_share_columns_total_pools_past_a_rows_div_zero
+
+improvise-b5dp, the second case the bead named. `Share = Revenue /
+SUM(Revenue)`: East's own `SUM` is 0, so its row draws `div/0`. The pooled
+total is 60/60 = **1.00**. It was green at `9948aaf5` and fails under the same
+rival as the test above.