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_observation_enriched.py

@@ -1051,7 +1051,8 @@ def test_observation_enriched_terrain_semantics_no_callable_no_share() -> None:
 def test_observation_enriched_e5_existing_keys_preserved() -> None:
     """Given: an Observation built for the L5 fields
     When: the top-level dict is inspected
-    Then: all pre-existing top-level keys remain (round, acting_combatant,
+    Then: all pre-existing top-level keys remain (schema_version, round,
+    acting_combatant,
     combatants, pairs, terrain, elevation) and the new keys
     (threatened, ranged_threatened, terrain_semantics,
     condition_definitions, initiative_order) are added — exact key set."""
@@ -1063,6 +1064,7 @@ def test_observation_enriched_e5_existing_keys_preserved() -> None:
 
     obs = _snapshot(engine, hero_state)
     assert set(obs) == {
+        "schema_version",
         "round",
         "acting_combatant",
         "combatants",
@@ -1138,7 +1140,7 @@ def _best_usable_attack(obs: Observation) -> dict[str, object] | None:
     return best
 
 
-def _enriched_llm_policy(obs: Observation, state: CombatantState) -> tuple[Action, ...]:
+def _enriched_llm_policy(obs: Observation) -> tuple[Action, ...]:
     """Read ONLY the observation — never state.combatant.* nor engine.*."""
     cast = _castable_spell_action(obs)
     if cast is not None: