feat: add ReactiveStatManager for markdown template support

Introduce ReactiveStatManager to allow dynamic stat replacement within
rendered markdown content. This component scans the article DOM for
`${key}` patterns and reactively updates them based on the journal
stream.

As part of this change, the SVG-based StatSheet system is removed in
favor of this more flexible text-based approach.

src/cli/completions/index.ts

@@ -14,7 +14,6 @@ export type {
   SparkTableCompletion,
   StatDef,
   StatTemplate,
-  StatSheet,
 } from "./types.js";
 
 /**
@@ -43,6 +42,5 @@ export function scanCompletions(
     sparkTables,
     stats: blocks.stats,
     statTemplates: blocks.statTemplates,
-    statSheets: blocks.statSheets,
   };
 }