feat: define action completion costs

docs/ECONOMIC_EVENTS.md

@@ -20,6 +20,11 @@ Events are immutable facts about completed transfers. They do not perform the
 transaction and are not replayed to reconstruct current state. Resource,
 inventory, and storage records remain authoritative.
 
+An action whose definition-backed completion cost becomes unavailable records
+a zero-amount `task_blocked` narrative fact with the action and shortfall
+reason. This makes late contention inspectable without pretending that a
+transfer occurred.
+
 ## Persistence and determinism
 
 `SimulationStateRecord` schema v3 stores the ordered event stream and
@@ -47,7 +52,6 @@ NPC record, so unloading presentation does not lose the fact.
 
 The stream is currently kept in full for the small simulation garden. Before
 large populations or long-running worlds, add measured retention,
-archival/summary rules, and query indexes. Denied attempts, depletion,
-witnesses, secrecy, causal links, and memories belong in later event/history
-slices; they should extend this record family without making prose
-authoritative.
+archival/summary rules, and query indexes. General denied attempts, witnesses,
+secrecy, causal links, and memories belong in later event/history slices; they
+should extend this record family without making prose authoritative.