Fix responsive DataGrid contraction

docs/DATAGRID_SIZING_CONTRACT.md

@@ -36,7 +36,8 @@ the column remains stopped until the pointer crosses the same boundary again.
 
 ## Persistence
 
-Only the pixel layout resulting from an explicit user resize is persisted.
+Only the pixel layout resulting from an explicit user resize is persisted,
+together with the container width at which the user selected it.
 Persisted widths are keyed by a signature containing column IDs, declared
 widths and bounds, resize affordances, sticky placement, initial fit, and resize
 behavior. A changed signature discards the old override and recomputes the
@@ -44,8 +45,13 @@ declared layout.
 
 Container reconciliation is suspended while a pointer drag is active. On
 release, the already-rendered pixel layout becomes the persisted preference.
-Reconciliation may grow it to prevent underflow, but never shrinks intentional
-user overflow, so there is no drag-end snap.
+Reconciliation at that same container width never shrinks intentional user
+overflow, so there is no drag-end snap. If the surrounding layout later
+contracts, persisted tracks may shrink toward their hard minima. The layout
+retains only the amount of horizontal overflow deliberately created by the
+user; an exact-cover layout therefore remains exact-cover at narrower widths.
+Legacy snapshots from the former hard-pixel persistence contract are discarded
+once and recomputed from the declared column layout.
 
 ## Regression Matrix
 
@@ -56,6 +62,7 @@ user overflow, so there is no drag-end snap.
 - hard-minimum horizontal overflow;
 - fixed-only cover grids;
 - persisted overrides under growth and viewport pressure;
+- responsive contraction of persisted layouts without losing deliberate overflow;
 - stale layout signatures;
 - first and middle-column right-side compensation;
 - last-resizable-column overflow, underflow stop, and reverse-pointer boundary;