refactor: model TreeNode as a discriminated union

Phase 3 (behavior-preserving):
- Replace the 45-optional-field TreeNode interface with a tagged union over
  category: OriginNode | ExperienceNode | ProjectNode | SkillNode | EducationNode,
  sharing a BaseNode. Category-specific fields now live only on their member.
- Modal template narrows by category (category === 'experience' && ...) instead
  of bare field-presence checks; equivalent at runtime since each field is
  exclusive to its category (proven by the union compiling against the data).
- stats utils narrow correctly: aggregateSkills filters to SkillNode[] via a type
  predicate; countByProficiency and getPlayerClass guard on category.

svelte-check: 0 errors (every data node conforms). lint/test/build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/utils/stats.ts

@@ -2,7 +2,7 @@
 // STATS UTILS — pure aggregation for the RPG-style character panel.
 // ==========================================================================
 
-import { treeNodes, type TreeNode } from '$lib/data';
+import { treeNodes, type SkillNode } from '$lib/data';
 
 /** Skill branch ids and their display labels (single source of truth). */
 export const BRANCH_IDS = [
@@ -40,17 +40,17 @@ export function calculatePlayerLevel(): number {
 
 /** Character "class" — the origin node's job title. */
 export function getPlayerClass(): string {
-	const originNode = treeNodes.find((node) => node.id === 'origin');
+	const originNode = treeNodes.find((node) => node.category === 'origin');
 	return originNode?.jobTitle || 'Software Engineer';
 }
 
 /** Group skill nodes by branch, sorted by proficiency. */
-export function aggregateSkills(): Record<string, { label: string; skills: TreeNode[] }> {
-	const result: Record<string, { label: string; skills: TreeNode[] }> = {};
+export function aggregateSkills(): Record<string, { label: string; skills: SkillNode[] }> {
+	const result: Record<string, { label: string; skills: SkillNode[] }> = {};
 
 	BRANCH_IDS.forEach((branchId) => {
 		const skills = treeNodes.filter(
-			(node) => node.parentId === branchId && node.category === 'skills'
+			(node): node is SkillNode => node.parentId === branchId && node.category === 'skills'
 		);
 
 		skills.sort((a, b) => {
@@ -74,7 +74,7 @@ export function countByProficiency(): {
 } {
 	const counts = { expert: 0, advanced: 0, intermediate: 0, beginner: 0 };
 	treeNodes.forEach((node) => {
-		if (node.proficiency) {
+		if (node.category === 'skills' && node.proficiency) {
 			counts[node.proficiency]++;
 		}
 	});