feat: implement scoped stat system with player/global support

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

src/components/journal/completions.ts

@@ -43,6 +43,7 @@ export interface StatDef {
   key: string;
   label: string;
   type: "number" | "string" | "enum" | "modifier" | "derived";
+  scope: "player" | "global";
   default?: string;
   target?: string;
   options?: string[];
@@ -204,10 +205,12 @@ function parseStatYaml(yaml: string, source: string): StatDef[] {
   function flushCurrent() {
     if (!current || !current.key) return;
     const type = (current.type || "number") as StatDef["type"];
+    const scope = (current.scope || "global") as StatDef["scope"];
     defs.push({
       key: current.key,
       label: current.label || current.key,
       type,
+      scope,
       default: current.default,
       target: current.target,
       options: