feat(engine): detect endings on every successful turn

After each state-mutating dispatch, evaluate world.endings in priority
order (true > wrong > bad). The first whose whenFlags are all satisfied
sets state.endedWith and appends a kind:'ending' transcript line. Once
ended, further dispatches return a "story has ended" narration.

Also update test-world fixtures and placeholder ending markdown files
to use whenFlags: { _never: true } instead of {} so that vacuously-true
empty flags don't accidentally fire on every successful turn.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/engine/encounters.test.ts

@@ -61,9 +61,9 @@ const world: World = {
     },
   },
   endings: {
-    true:  { whenFlags: {}, narration: '' },
-    wrong: { whenFlags: {}, narration: '' },
-    bad:   { whenFlags: {}, narration: '' },
+    true:  { whenFlags: { _never: true }, narration: '' },
+    wrong: { whenFlags: { _never: true }, narration: '' },
+    bad:   { whenFlags: { _never: true }, narration: '' },
   },
 }