State each closed set in one place: healing services and the morale exempt code

[mmacy]

Two places where a closed set is stated twice or a code carries two meanings, both found by reviewers during the docstring pass.

1. PurchaseHealing.service is a Literal in crawl/commands.py and HEALING_SERVICES in crawl/exploration.py is a dict with the same keys and the prices. Adding a service to one does nothing without the other, and the docstring on HEALING_SERVICES had to warn about it (Docstrings: crawl exploration #87). Derive one from the other, or make the service table the single source and validate the command against it.

2. MoraleCheckedEvent emits one code, combat.morale.exempt, for two opposite outcomes: a score of 2 or less never holds and a score of 12 or more always holds. The score field is the only discriminator, so a consumer cannot render the outcome from the code alone, which every other event in the catalog lets it do (Docstrings: core effects and events #95). Two codes, or an outcome field the code already carries elsewhere.

Both are additive within the schema version: a new code and a derived table change no existing field. Update the generated message-code and rejection-code pages in the same change.