fix(01-02): TS-strict narrowing for EvalResult discriminated union

Rule 1 deviation: TS strict mode (noImplicitAny + strict) couldn't narrow
the EvalResult union via 'r.ok' direct access because the {unknown:true,raw}
variant has no 'ok' property. Added explicit type-guard helpers (isUnknown,
isOk, isFail) for both production and spec narrowing. Runtime behavior
unchanged; tsc --noEmit now exits clean for the leveling lib.

Files:
- prereq-evaluator.ts: 3 type-guard functions, used in OR/AND walkers
- prereq-evaluator.spec.ts: isFail() in lieu of result.ok=== checks

server/src/modules/leveling/lib/prereq-evaluator.ts

@@ -291,6 +291,18 @@ type NodeResult =
   | { ok: false; reason: string }
   | { unknown: true; raw: string };
 
+function isUnknown(r: NodeResult): r is { unknown: true; raw: string } {
+  return 'unknown' in r;
+}
+
+function isOk(r: NodeResult): r is { ok: true } {
+  return 'ok' in r && r.ok === true;
+}
+
+function isFail(r: NodeResult): r is { ok: false; reason: string } {
+  return 'ok' in r && r.ok === false;
+}
+
 function evaluateNode(node: Node, ctx: CharacterContext, raw: string): NodeResult {
   if (node.kind === 'atom') {
     return evaluateAtom(node.atom, ctx);
@@ -300,12 +312,12 @@ function evaluateNode(node: Node, ctx: CharacterContext, raw: string): NodeResul
     let firstFailReason: string | null = null;
     for (const child of node.children) {
       const r = evaluateNode(child, ctx, raw);
-      if ('unknown' in r && r.unknown) {
+      if (isUnknown(r)) {
         // UNKNOWN-aggressive: any unknown atom poisons the whole prereq.
         return { unknown: true, raw };
       }
-      if (r.ok) return { ok: true };
-      if (firstFailReason === null && !r.ok) {
+      if (isOk(r)) return { ok: true };
+      if (firstFailReason === null && isFail(r)) {
         firstFailReason = r.reason;
       }
     }
@@ -318,10 +330,10 @@ function evaluateNode(node: Node, ctx: CharacterContext, raw: string): NodeResul
   // AND
   for (const child of node.children) {
     const r = evaluateNode(child, ctx, raw);
-    if ('unknown' in r && r.unknown) {
+    if (isUnknown(r)) {
       return { unknown: true, raw };
     }
-    if (!r.ok) return r;
+    if (isFail(r)) return r;
   }
   return { ok: true };
 }