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/journal/index.ts

@@ -51,5 +51,16 @@ export { DynamicForm } from "./DynamicForm";
 export { useJournalCompletions, invalidateCompletions } from "./completions";
 export { dispatchCommand } from "./command-dispatcher";
 export type { DispatchContext, DispatchResult } from "./command-dispatcher";
+export { parseInput } from "./command-parser";
+export type { ParsedInput, CompletionItem } from "./command-parser";
+export { buildCompletions } from "./command-completions";
+export type { CompletionsContext } from "./command-completions";
+export { VariableView } from "./VariableView";
+export { parseDeclareCsv } from "./declare-parser";
+export type { VarDeclaration, TagModifier } from "./declare-parser";
+export { evaluateExpression, expressionIsTag } from "./variable-expression";
+export type { EvalContext, EvalResult } from "./variable-expression";
+export { initReactivity, computeCascade, computeActiveTags, extractDependencies } from "./var-reactivity";
+export type { VarReactivityState, VariableStore } from "./var-reactivity";
 export { JournalContext, useJournalContext } from "./JournalContext";
 export type { JournalContextValue } from "./JournalContext";