refactor(#74) P3: add the 8 ability modules + ABILITIES registry

Each per-feature LLM ability is now a self-contained module in
server/abilities/ with its own Ability<K> const (schema line, constraint
bullet, validator, applier, isMeaningful, stub default, user-payload
field). The ordered ABILITIES registry (index.ts) assembles them in the
canonical order: takeovers → wars → hybrid → economy → industry → nuclear
→ leaders → ideology.

All function bodies are moved VERBATIM from engine.ts. The engine is
untouched in this commit — P4 will rewire it to use the registry.

server/abilities/shared.ts

@@ -94,3 +94,16 @@ export const NEW_NATION_STATS: NationStats = {
   stability: 50,
   population: 50,
 };
+
+/**
+ * Coerce an unknown LLM JSON value to a trimmed, non-empty string, or `null`
+ * when it is not a non-empty string. An optional `max` caps the length (used
+ * for the 200-char caps on nationOp fields). Shared by every defensive
+ * validator (engine + the ability modules). Never throws.
+ */
+export function str(s: unknown, max = 0): string | null {
+  if (typeof s !== 'string') return null;
+  const t = s.trim();
+  if (!t) return null;
+  return max ? t.slice(0, max) : t;
+}