Relax INV-1/INV-2 in 001-advance-turn spec to allow empty encounters

Prepares for 002-add-combatant by treating an empty combatant list as
a valid aggregate state (activeIndex must be 0). AdvanceTurn behavior
on non-empty encounters is unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/001-advance-turn/spec.md

@@ -82,11 +82,20 @@ domain events.
 
 ### Edge Cases
 
-- Empty combatant list: AdvanceTurn MUST reject with an error.
+- Empty combatant list: valid aggregate state, but AdvanceTurn MUST
+  return a DomainError (no state change, no events).
 - Single combatant: every advance wraps and increments the round.
 - Large round numbers: no overflow or special-case behavior; round
   increments uniformly.
 
+## Clarifications
+
+### Session 2026-03-03
+
+- Q: Should an encounter with zero combatants be a valid aggregate state? → A: Yes. Empty encounter is valid; AdvanceTurn returns DomainError.
+- Q: What is activeIndex when combatants list is empty? → A: activeIndex MUST be 0.
+- Q: Does this change any non-empty encounter behavior? → A: No. All existing acceptance scenarios and event contracts remain unchanged.
+
 ## Domain Model *(mandatory)*
 
 ### Key Entities
@@ -113,10 +122,13 @@ MUST be verified by tests.
 
 ### Invariants
 
-- **INV-1**: An encounter MUST have at least one combatant.
-  Operations on an empty encounter MUST fail.
-- **INV-2**: activeIndex MUST always satisfy
-  0 <= activeIndex < len(combatants).
+- **INV-1**: An encounter MAY have zero combatants (an empty
+  encounter is a valid aggregate state). AdvanceTurn on an empty
+  encounter MUST return a DomainError with no state change and no
+  events.
+- **INV-2**: If combatants.length > 0, activeIndex MUST satisfy
+  0 <= activeIndex < combatants.length. If combatants.length == 0,
+  activeIndex MUST be 0.
 - **INV-3**: roundNumber MUST be a positive integer (>= 1) and MUST
   only increase (never decrease or reset).
 - **INV-4**: AdvanceTurn MUST be a pure function of the current