refactor: implement base/modifier separation for variable reactivity

Introduce a distinction between base values (set via `/set` or
declarations) and active modifiers (applied via tags). This ensures
tag transitions and numeric modifiers are calculated correctly during
cascades.

- Refactor `VariableView` to use a flat list with hover popups for
  declarations and active modifiers.
- Update `command-dispatcher` to use `getCombined` for accurate
  value lookups and `setBase` for variable updates.
- Update `var-reactivity` to manage `baseValues`, `activeMods`, and
  `sourceActivations` separately.
- Export new utility functions for accessing combined values,
  modifiers, and declaration expressions.

src/components/journal/index.ts

@@ -60,7 +60,7 @@ 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, computeInitialValues } from "./var-reactivity";
+export { initReactivity, computeCascade, getCombined, getMods, getDeclExpr, setBase, extractDependencies, computeInitialValues } from "./var-reactivity";
 export type { VarReactivityState, VariableStore } from "./var-reactivity";
 export { JournalContext, useJournalContext } from "./JournalContext";
 export type { JournalContextValue } from "./JournalContext";