improvise-h6z1: one fold memo per grid build, keyed by fold key and span

viewport::grid_body makes one FoldMemo and passes it to every cell and
total through GridLayout::evaluate_in / column_total, so a fold row's
formula chain is evaluated once across the row (9, not 45). The bench
gains a folded five-formula chain case; stored-only grids are unchanged.

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

context/code/core.md

@@ -929,8 +929,18 @@ every member. Five decisions:
     without the memo, about 60 ms with it, and about 10 ms on gh/main. gh/main
     was cheap because it read the members out of the formula cache, which is
     what gave the wrong ratios. The remaining ~6× is each column's cell
-    re-deriving the chain below it (45 evaluations per row where 9 would do). A
-    memo scoped to the whole grid would remove it; it is not built.
+    re-deriving the chain below it (45 evaluations per row where 9 would do).
+    **The memo is now shared across one grid build** (improvise-h6z1).
+    `viewport::grid_body` makes one `FoldMemo` and passes it through
+    `GridLayout::evaluate_in` and `column_total` for every cell and total, so a
+    fold row's chain is evaluated 9 times, not 45. A memo is valid only for the
+    model state it read. That is why it is made per build and never stored on
+    `GridLayout`, which outlives cell writes.
+    The memo is keyed by the fold key **and** the span's categories. Within one
+    grid build the span half cannot yet differ for one key, because a cell's key
+    always names the row categories and a total's never does. It is kept because
+    `evaluate_spanning_in` is public, and a caller sharing one memo across two
+    spans would otherwise read the wrong fold. No test can see it today.
 - **A fold covers the members the grid draws** (improvise-f2oo). Hidden members
   are not in it. So a fold is exactly the rows it replaces, the total of a
   column's total does not move on `z`, and a group whose every member is hidden