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_llm_engine_capabilities.py

@@ -1,7 +1,7 @@
 """Package U8 — final validation: an observation-only LLM policy drives the
 Phase-2 capabilities (U1-U7b) end-to-end through the real engine.
 
-Each test wires a ``llm_policy`` (``LLMPolicy(observation, state)``) that
+Each test wires a ``llm_policy`` (``LLMPolicy(observation)``) that
 reads ONLY the frozen Observation snapshot — never ``engine.*`` — and emits
 one newly-unblocked tactical decision per capability: ``ready`` (U4),
 ``delay`` (U3), ``total_defense`` (U5a), ``move`` with an explicit
@@ -121,7 +121,7 @@ def _counting(decide: Decide) -> LLMPolicy:
     """LLMPolicy calling ``decide(id, call_no, obs)`` per turn (obs only)."""
     calls: dict[str, int] = {}
 
-    def policy(obs: Observation, _state: CombatantState) -> tuple[Action, ...]:
+    def policy(obs: Observation) -> tuple[Action, ...]:
         cid = str(_me(obs)["id"])
         n = calls.get(cid, 0)
         calls[cid] = n + 1