feat(phase2): observation-only LLMPolicy entry (D1) + pinned schema_version (P0.1)

- LLMPolicy is now Callable[[Observation], tuple[Action, ...]]: the snapshot
  is the policy's sole information source; the live CombatantState is no
  longer passed to new-style policies.
- CombatEngine._llm_adapter keeps a compatibility path for pre-D1
  two-argument callables: the signature is inspected ONCE at wrap time
  (_accepts_state) and legacy policies still receive (observation, state).
- Observation.to_dict() carries schema_version (OBSERVATION_SCHEMA_VERSION=1)
  as its first pinned key; bump only on removal/meaning change.
- Pinned keyset tests updated (test_observation, test_observation_enriched);
  new tests: observation-only e2e, legacy 2-arg e2e through the adapter,
  schema_version pin + JSON-serializability.
- README: documents the new entry signature and the schema_version key.

tests/test_action_feedback.py

@@ -229,7 +229,7 @@ def test_results_visible_in_next_turn_observation() -> None:
     # the second invocation must already see the first turn's rejection.
     seen: list[Observation] = []
 
-    def llm_policy(obs: Observation, state: CombatantState) -> tuple[Action, ...]:
+    def llm_policy(obs: Observation) -> tuple[Action, ...]:
         seen.append(obs)
         if len(seen) == 1:
             return (Action(kind="move"),)