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/components/SkillTree.svelte

@@ -1240,7 +1240,7 @@
 				{/if}
 
 				<!-- Experience-specific content -->
-				{#if selectedNode.company}
+				{#if selectedNode.category === 'experience' && selectedNode.company}
 					<div class="meta-info">
 						<span class="company">{selectedNode.company}</span>
 						{#if selectedNode.startDate}
@@ -1252,7 +1252,7 @@
 				{/if}
 
 				<!-- Education-specific content -->
-				{#if selectedNode.institution}
+				{#if selectedNode.category === 'education' && selectedNode.institution}
 					<div class="meta-info">
 						<span class="company">{selectedNode.institution}</span>
 						{#if selectedNode.degree}
@@ -1265,7 +1265,7 @@
 				{/if}
 
 				<!-- Skill-specific content -->
-				{#if selectedNode.proficiency}
+				{#if selectedNode.category === 'skills' && selectedNode.proficiency}
 					<div class="meta-info">
 						<span class="proficiency-badge {selectedNode.proficiency}"
 							>{selectedNode.proficiency}</span
@@ -1281,7 +1281,7 @@
 				{/if}
 
 				<!-- Highlights (for experience) -->
-				{#if selectedNode.highlights && selectedNode.highlights.length > 0}
+				{#if selectedNode.category === 'experience' && selectedNode.highlights && selectedNode.highlights.length > 0}
 					<ul class="highlights">
 						{#each selectedNode.highlights as highlight (highlight)}
 							<li>{highlight}</li>
@@ -1290,7 +1290,7 @@
 				{/if}
 
 				<!-- Tech stack (for projects) -->
-				{#if selectedNode.techStack && selectedNode.techStack.length > 0}
+				{#if selectedNode.category === 'projects' && selectedNode.techStack && selectedNode.techStack.length > 0}
 					<div class="tech-stack">
 						{#each selectedNode.techStack as tech (tech)}
 							<span class="tech-tag">{tech}</span>
@@ -1299,14 +1299,14 @@
 				{/if}
 
 				<!-- Project status badge -->
-				{#if selectedNode.status}
+				{#if selectedNode.category === 'projects' && selectedNode.status}
 					<div class="project-status">
 						<span class="status-badge {selectedNode.status}">{selectedNode.status}</span>
 					</div>
 				{/if}
 
 				<!-- Links (for projects) -->
-				{#if selectedNode.projectUrl || selectedNode.repoUrl || selectedNode.pypiUrl || selectedNode.npmUrl}
+				{#if selectedNode.category === 'projects' && (selectedNode.projectUrl || selectedNode.repoUrl || selectedNode.pypiUrl || selectedNode.npmUrl)}
 					<div class="links">
 						{#if selectedNode.projectUrl}
 							<a

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

src/lib/utils/stats.test.ts

@@ -44,14 +44,14 @@ describe('countByProficiency', () => {
 	it('sums to the total number of nodes that have a proficiency', () => {
 		const counts = countByProficiency();
 		const total = Object.values(counts).reduce((sum, n) => sum + n, 0);
-		const expected = treeNodes.filter((n) => n.proficiency).length;
+		const expected = treeNodes.filter((n) => n.category === 'skills' && n.proficiency).length;
 		expect(total).toBe(expected);
 	});
 });
 
 describe('getPlayerClass', () => {
 	it("returns the origin node's job title", () => {
-		const origin = treeNodes.find((n) => n.id === 'origin');
+		const origin = treeNodes.find((n) => n.category === 'origin');
 		expect(getPlayerClass()).toBe(origin?.jobTitle);
 	});
 });

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]++;
 		}
 	});