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/completions/index.ts

@@ -4,16 +4,22 @@
 
 import { diceSource } from "./sources/dice.js";
 import { linksSource } from "./sources/links.js";
+import { sparkTablesSource } from "./sources/spark-tables.js";
 import type { CompletionSource, CompletionsPayload } from "./types.js";
 
 export type {
   CompletionsPayload,
   DiceCompletion,
   LinkCompletion,
+  SparkTableCompletion,
 } from "./types.js";
 
 /** Registered sources — open for extension */
-const sources: CompletionSource[] = [diceSource, linksSource];
+const sources: CompletionSource[] = [
+  diceSource,
+  linksSource,
+  sparkTablesSource,
+];
 
 /**
  * Scan the full content index and return structured completion data.