feat: add spark table completion and rolling support

Implement "spark tables" functionality, which allows users to roll
dice against markdown tables to retrieve specific values.

- Add `sparkTablesSource` to scan markdown files for tables starting
  with a dice notation (e.g., d6, d20).
- Implement `/spark` command in the journal to resolve and roll
  spark tables.
- Add a new message type `spark` with a dedicated UI component to
  render the results.
- Update completions API to include spark table metadata.

src/cli/commands/serve.ts

@@ -227,13 +227,17 @@ export function createContentServer(
   host: string = "0.0.0.0",
 ): ContentServer {
   let contentIndex: ContentIndex = {};
-  let completionsIndex: CompletionsPayload = { dice: [], links: [] };
+  let completionsIndex: CompletionsPayload = {
+    dice: [],
+    links: [],
+    sparkTables: [],
+  };
 
   /** 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}`,
+      `[completions] dice=${completionsIndex.dice.length} links=${completionsIndex.links.length} sparkTables=${completionsIndex.sparkTables.length}`,
     );
   }