whatif.py: add --remove for teardown/demolish scenarios

Mirrors --add - removes AMOUNT (default: all) of a building from the
copied spec. Used this to check whether BHP's HWP is still worth
running after its price drop (still +19.8k-28.4k/day marginal,
nowhere near a teardown call) without hand-writing two spec files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_whatif.py

@@ -23,6 +23,18 @@ def test_apply_add_appends_new_building_with_recipe():
     assert spec["buildings"] == [{"building": "CHP", "amount": 1, "recipes": ["MG,S,CU=>IND"]}]
 
 
+def test_apply_remove_drops_building_entirely_by_default():
+    spec = {"buildings": [{"building": "HWP", "amount": 1, "recipes": ["AL,STL,HE=>BHP"]}]}
+    whatif.apply_remove(spec, "HWP")
+    assert spec["buildings"] == []
+
+
+def test_apply_remove_partial_amount():
+    spec = {"buildings": [{"building": "CHP", "amount": 3, "recipes": ["MG,S,CU=>IND"]}]}
+    whatif.apply_remove(spec, "CHP:1")
+    assert spec["buildings"] == [{"building": "CHP", "amount": 2, "recipes": ["MG,S,CU=>IND"]}]
+
+
 def test_apply_infra_adds_delta():
     spec = {"infrastructure": {"HB1": 4}}
     whatif.apply_infra(spec, "HB1:+1,HB2:+2")