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/data.ts

@@ -1,49 +1,20 @@
-export interface TreeNode {
+// Fields shared by every node, regardless of category.
+interface BaseNode {
 	id: string;
 	label: string;
 	description: string;
 	x: number;
 	y: number;
 	parentId: string | null; // null means it's the root or connects to center
-	category: 'origin' | 'experience' | 'projects' | 'skills' | 'education';
 	size: 'x-large' | 'large' | 'medium' | 'small';
 	icon?: string;
+}
 
-	// ========== EXPERIENCE-SPECIFIC FIELDS ==========
-	company?: string;
-	jobTitle?: string;
-	startDate?: string;
-	endDate?: string; // Use "Present" for current role
-	highlights?: string[]; // Key achievements or responsibilities
-
-	// ========== CONTRACT-SPECIFIC FIELDS ==========
-	contractName?: string;
-	client?: string;
-	contractStart?: string;
-	contractEnd?: string;
-
-	// ========== PROJECT-SPECIFIC FIELDS (extended) ==========
-	techStack?: string[];
-	projectUrl?: string;
-	repoUrl?: string;
-	npmUrl?: string; // For npm packages
-	pypiUrl?: string; // For PyPI packages
-	downloads?: string; // "1,000+ downloads"
-	stars?: number; // GitHub stars
-	status?: 'active' | 'maintained' | 'archived' | 'completed';
-
-	// ========== SKILL-SPECIFIC FIELDS ==========
-	proficiency?: 'beginner' | 'intermediate' | 'advanced' | 'expert';
-	yearsOfExperience?: number;
-
-	// ========== EDUCATION-SPECIFIC FIELDS ==========
-	institution?: string;
-	degree?: string;
-	field?: string;
-	graduationYear?: string;
-
-	// ========== ORIGIN-SPECIFIC FIELDS ==========
+// Center node with profile/contact info.
+export interface OriginNode extends BaseNode {
+	category: 'origin';
 	fullName?: string;
+	jobTitle?: string;
 	aboutMe?: string;
 	location?: string;
 	email?: string;
@@ -53,6 +24,52 @@ export interface TreeNode {
 	resumeUrl?: string;
 }
 
+// Jobs and contract roles.
+export interface ExperienceNode extends BaseNode {
+	category: 'experience';
+	company?: string;
+	jobTitle?: string;
+	startDate?: string;
+	endDate?: string; // Use "Present" for current role
+	highlights?: string[]; // Key achievements or responsibilities
+	contractName?: string;
+	client?: string;
+	contractStart?: string;
+	contractEnd?: string;
+}
+
+// Personal projects / packages.
+export interface ProjectNode extends BaseNode {
+	category: 'projects';
+	techStack?: string[];
+	projectUrl?: string;
+	repoUrl?: string;
+	npmUrl?: string; // For npm packages
+	pypiUrl?: string; // For PyPI packages
+	downloads?: string; // "1,000+ downloads"
+	stars?: number; // GitHub stars
+	status?: 'active' | 'maintained' | 'archived' | 'completed';
+}
+
+// Technical skills with proficiency.
+export interface SkillNode extends BaseNode {
+	category: 'skills';
+	proficiency?: 'beginner' | 'intermediate' | 'advanced' | 'expert';
+	yearsOfExperience?: number;
+}
+
+// Academic background.
+export interface EducationNode extends BaseNode {
+	category: 'education';
+	institution?: string;
+	degree?: string;
+	field?: string;
+	graduationYear?: string;
+}
+
+// Discriminated union over `category` — narrow on it to access category fields.
+export type TreeNode = OriginNode | ExperienceNode | ProjectNode | SkillNode | EducationNode;
+
 // The center of our tree is at (0, 0)
 // Positive X = right, Negative X = left
 // Positive Y = down, Negative Y = up