docs: the ambiguous-date heuristic is a ruled limit, not an open question

improvise-l8rh carries the user's ruling of 2026-09-20: leave the first-match
heuristic as it is and track it as a known limit. The prose still read as a
filed-and-open question, and it did not record what the ruling rests on -- that
a day-first default only moves which half of the world imports silently wrong,
that the honest fix changes FieldProposal's shape, and that the limit is
tolerable chiefly because improvise-7yx made it deterministic.

No behaviour change; the two pinning tests are unchanged.

improvise-l8rh

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

context/code/formula-and-io.md

@@ -424,7 +424,21 @@ and the derived `_Month`/`_Quarter` categories inherit the error. What makes it
 tolerable is that the wizard shows the detected format and lets the user change
 it — the same argument two-digit years already rested on. It is pinned by a test
 so that changing the answer is a decision somebody makes rather than a side
-effect of reordering the table. Filed as **improvise-l8rh**.
+effect of reordering the table.
+
+**Ruled a known limit** (improvise-l8rh, the user, 2026-09-20). Two things the
+ruling rests on. Changing the default to day-first only moves which half of the
+world imports silently wrong; and the honest fix is larger than the heuristic —
+an ambiguous column should *surface* its ambiguity rather than resolve it, which
+changes `FieldProposal`'s shape and the wizard step that displays it, the way
+`Dataset::warnings` returns a decision the user should hear about as data. Worth
+doing deliberately, not as a patch.
+
+What makes the limit reasonable to live with is that it is now *deterministic*:
+`analyze_records` iterated a `HashSet` until improvise-7yx, so the ten samples
+`detect_date_format` inspects were a different ten every run and a column whose
+eleventh value ruled out `%m/%d/%Y` detected a different format run to run. The
+ten-sample window itself is unchanged and is part of the limit.
 
 ### CSV parsing
 

context/test-evidence/formula-and-io.md

@@ -461,9 +461,10 @@ can override it. The same argument as the ambiguity below.
 nothing in the data can tell them apart. The heuristic is first-match over
 `DATE_FORMATS`, so the American reading wins — a European CSV imports with its
 days and months swapped, silently, and the derived `_Month`/`_Quarter`
-categories inherit the error. Filed as **improvise-l8rh**; pinned here so that
-changing the answer is a decision somebody makes rather than a side effect of
-reordering the table.
+categories inherit the error. **improvise-l8rh ruled this a known limit** rather
+than a defect, so this test is what makes changing the answer a decision
+somebody makes rather than a side effect of reordering the table. The argument
+is in `context/code/formula-and-io.md`.
 
 The loop at the end is the precondition that makes this a statement about
 *ambiguity* rather than about one format merely fitting: the other reading has