Introduces a scoping mechanism for statistics, allowing properties
to be defined as either "player" (prefixed with player name) or
"global".
- Adds `scope` property to StatDef
- Implements key resolution to handle bare keys vs full keys
- Updates `canModifyStat` to enforce player-specific permissions
- Enhances `makeStatLookup` to resolve scoped references in formulas
- Adds documentation for the new stat system
