requirements for clearing stalled belts

docs/requirements.md

@@ -65,7 +65,7 @@ The following config files drive game parameters:
 - REQ-MAT-1: Materials are transported exclusively via belts and splitters.
 - REQ-MAT-2: A building accepts items from all adjacent belt tiles whose direction points toward the building, provided the item is an input required by the currently selected recipe.
 - REQ-MAT-3: Each building has one fixed output port (direction determined by rotation). Produced items are placed onto the belt at the output port.
-- REQ-MAT-4: Each building's input buffer holds up to twice the quantity of each required input material for one production cycle.
+- REQ-MAT-4: Each building's input buffer holds up to twice the quantity of each required input material for one production cycle. When the player selects a new recipe or blueprint, all items in the input buffer are cleared.
 - REQ-MAT-5: Each building has an output buffer that holds up to twice the quantity produced by one production cycle. If the output buffer is full, production stops until space is available.
 - REQ-MAT-6: The building blocks stock is the only global inventory. All other materials exist only in building buffers or on belt tiles.
 
@@ -155,9 +155,10 @@ The screen is divided into three vertical sections:
 - REQ-UI-7: The player selects multiple buildings by box-drag or by Ctrl+clicking individual buildings to add or remove them from the selection.
 - REQ-UI-8: When multiple buildings are selected, the panel shows how many of each building type are selected. No per-building detail is shown.
 - REQ-UI-9: Recipe, blueprint, ship stance, and target priority configuration for a selected building is shown and changed inline within this panel.
+- REQ-UI-10: When one or more belt or splitter tiles are selected, the panel shows a "Clear" button that removes all items from the selected tiles. This can be used to resolve stalled belts and splitters.
 
 ### Build Button Grid
 
-- REQ-UI-10: All placeable building types are shown as a flat grid of buttons with no grouping.
-- REQ-UI-11: Each button caption shows the building name and its building block cost, e.g. "Belt: 2 Blocks".
-- REQ-UI-12: Buttons for buildings the player cannot currently afford are shown as disabled.
+- REQ-UI-11: All placeable building types are shown as a flat grid of buttons with no grouping.
+- REQ-UI-12: Each button caption shows the building name and its building block cost, e.g. "Belt: 2 Blocks".
+- REQ-UI-13: Buttons for buildings the player cannot currently afford are shown as disabled.