feat: add stat completion source

Implement a new completion source that extracts stat definitions
from markdown files using `yaml` or `csv` code blocks with
`role=stat`.

src/cli/commands/serve.ts

@@ -271,13 +271,14 @@ export function createContentServer(
     dice: [],
     links: [],
     sparkTables: [],
+    stats: [],
   };
 
   /** Re-scan completions from current content index (cached) */
   function recomputeCompletions(): void {
     completionsIndex = scanCompletions(contentIndex);
     console.log(
-      `[completions] dice=${completionsIndex.dice.length} links=${completionsIndex.links.length} sparkTables=${completionsIndex.sparkTables.length}`,
+      `[completions] dice=${completionsIndex.dice.length} links=${completionsIndex.links.length} sparkTables=${completionsIndex.sparkTables.length} stats=${completionsIndex.stats.length}`,
     );
   }