Ingot molds drop bars out of the bottom on their own

Bars no longer get thrown up into the furnace, and F is no longer needed to
collect them. Blocked bars wait in the trough; the short stub stays for the
next firing; F is optional upkeep (clean, chip slag, rake).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -46,7 +46,7 @@ decides something new, add it under "Decisions" with the date.
 2. Vacuum up what you want (tick materials to take only those), and blow it out where you need it.
 3. Build a furnace from wall blocks, with a grate floor over a mold. Charge it with ore and coal,
    light it, and let the molten metal run into the mold.
-4. Knock out bars and sell them at the trade dock, or fill buyers' orders, which pay more.
+4. Bars drop out of the bottom of the mold; collect them and sell them at the trade dock, or fill buyers' orders, which pay more.
 5. Better bits (copper, then iron, then steel) open harder rock: granite, then basalt. Deeper
    ores give tin, zinc, silver and gold, and alloys (bronze, brass) pay more.
 6. Automate with belts, screw tubes, filters, fans, pipes and machines. Pay off the claim.
@@ -77,7 +77,10 @@ Details and numbers live in the code and in `docs/systems/`.
   - Blocks: walls of any of several materials, belts, screw tubes, filters, grates, gates,
     pipes, pumps, outlets.
   - Machines: fan, Refiner, Stone cutter, Silo.
-  - Molds: ingot, bit, tank.
+  - Molds: ingot, bit, tank. Ingot bars drop out of the bottom onto whatever is under the mold;
+    leave one open block below and a belt under that carries them off. F is optional upkeep: it
+    cleans the trough, chips off slag and rakes the furnace grate. Short stubs stay for the next
+    firing.
   - Build actions: drag for lines, Shift-drag for rectangles, Ctrl to replace, hold C / X and
     drag to copy / cut, V to paste, Ctrl+Z to undo, middle mouse to pick,
     Shift+Q/E to switch build tabs.
@@ -132,6 +135,9 @@ above, including:
   does not replace it.
 
 **2026-10-23:**
+- **Ingot molds drop bars below automatically.** The owner reported bars being thrown into the
+  furnace above and wants belts underneath to carry them off. Blocked bars wait in the trough;
+  short stubs stay for the next firing. F stays available for cleaning and raking.
 - **Screw tubes carry no more than belts:** they move every other tick. For more throughput,
   build more lines.
 - **A heap on a belt moves as one**, up to 96 grains deep, not just its bottom layer. The owner