refactor: extract PixiJS rendering into pixi/ modules

Phase 5 (behavior-preserving): decompose the ~1000-line onMount into focused
modules coordinated by a plain (non-reactive) controller object:
- pixi/types.ts        LineObject / NodeObject / BackgroundLayers
- pixi/controller.ts   shared mutable pan/zoom state + Pixi refs (NOT $state,
                       which would break PixiJS object identity)
- pixi/background.ts   gradient/nebula/stars/dust/vignette + ambient loops
- pixi/lines.ts        plasma-beam connections
- pixi/nodes.ts        origin + regular nodes; collapses the duplicated pointer
                       handlers into one attachNodeInteraction helper
- pixi/interaction.ts  pan/zoom/parallax + keyboard nav (return cleanups)
- pixi/animation.ts    sequenced entrance animation

onMount is now a thin orchestrator. SkillTree.svelte: 1177 -> 274 lines
(2453 at the start of the refactor). Listener passivity, the hasMoved click
guard, eventMode-until-animated ordering, and the R-reset parallax rebaseline
are all preserved.

svelte-check: 0 errors/0 warnings. lint/test/build clean. Verified onMount runs
with no runtime exceptions (background + stats panel render). NOTE: pan/zoom/
node-click/keyboard need a manual real-browser check (headless can't paint nodes).

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

src/lib/pixi/controller.ts

@@ -0,0 +1,58 @@
+// Mutable controller object shared across the PixiJS modules.
+//
+// IMPORTANT: this must NOT be wrapped in Svelte's `$state` — the deep proxy
+// breaks PixiJS's internal object identity checks. Keep it a plain object so
+// the interaction, node, and animation modules can read/write the same
+// mutable pan/zoom state by reference (the same single source of truth the
+// original onMount closure relied on).
+import type { Application, Container } from 'pixi.js';
+import type { BackgroundLayers, LineObject, NodeObject } from './types';
+
+export interface SkillTreeController {
+	app: Application;
+	treeContainer: Container;
+	bg: BackgroundLayers;
+	nodeObjects: NodeObject[];
+	lineObjects: LineObject[];
+
+	// Pan/zoom mutable state (was closure vars in onMount).
+	isDragging: boolean;
+	hasMoved: boolean;
+	dragStartX: number;
+	dragStartY: number;
+	containerStartX: number;
+	containerStartY: number;
+	lastTouchDistance: number;
+	isTouchZooming: boolean;
+
+	// Parallax baseline (re-set on view reset) + final zoom scale.
+	initialTreeX: number;
+	initialTreeY: number;
+	endScale: number;
+}
+
+export function createController(
+	app: Application,
+	treeContainer: Container,
+	bg: BackgroundLayers,
+	endScale: number
+): SkillTreeController {
+	return {
+		app,
+		treeContainer,
+		bg,
+		nodeObjects: [],
+		lineObjects: [],
+		isDragging: false,
+		hasMoved: false,
+		dragStartX: 0,
+		dragStartY: 0,
+		containerStartX: 0,
+		containerStartY: 0,
+		lastTouchDistance: 0,
+		isTouchZooming: false,
+		initialTreeX: treeContainer.x,
+		initialTreeY: treeContainer.y,
+		endScale
+	};
+}