NodeBuilder: Rename .monitor to .observer

[sunag]

Description

Rename .monitor to .observer to maintain consistency

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.25 78.28	336.25 78.28	+0 B +0 B
WebGPU	503.38 139.83	503.39 139.83	+5 B +3 B
WebGPU Nodes	502.85 139.72	502.85 139.73	+5 B +2 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.25 112.13	465.25 112.13	+0 B +0 B
WebGPU	577.16 156.46	577.16 156.47	+5 B +1 B
WebGPU Nodes	532.54 146.02	532.55 146.02	+5 B +3 B