feat(observation): expose spell cast time in the snapshot

The per-spell Observation dict gains the additive cast_time key: the
registry spell's budget code on a hit, "standard" on a miss (mirroring
the engine's resolver fallback), so an LLM policy can schedule casts
against the action budget it sees reported back.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_observation_enriched.py

@@ -532,6 +532,7 @@ def test_observation_enriched_spells_unknown_graceful() -> None:
             "half_on_save": False,
             "save_negates": False,
             "area": None,
+            "cast_time": "standard",
         },
     )