refactor: replace stat system with variable system

Replaces the specialized `stat` system with a more general-purpose
variable system. This includes:

- Renaming `ReactiveStatManager` to `ReactiveVariableManager`
- Changing template syntax from `${key}` to `{{$key}}`
- Replacing `StatsView` with `VariableView` to show declared, plain,
  and tag-typed variables
- Updating command `/stat` to `/set`
- Refactoring the reactivity engine to support variable declarations
  and tag modifiers via `csv role=declare` blocks

src/components/index.ts

@@ -16,7 +16,7 @@ export { Article } from "./Article";
 export type { ArticleProps } from "./Article";
 export { RevealManager } from "./RevealManager";
 export { CommandLinkManager } from "./CommandLinkManager";
-export { ReactiveStatManager } from "./journal/ReactiveStatManager";
+export { ReactiveVariableManager } from "./journal/ReactiveVariableManager";
 export { MobileSidebar, DesktopSidebar } from "./Sidebar";
 export type { SidebarProps } from "./Sidebar";
 export { FileTreeNode, HeadingNode } from "./FileTree";