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
