feat: polish Layers DevTools UI and add playground #69

Job	Run time
Gate (skip full CI for version PRs?)	3s
✅ Typecheck	57s
🧰 Build	29s
🌐 Test (DOM)	1m 2s
📖 Docs site (Blume)	1m 23s
🕵 Lint	14s
🔬 Test	30s
💅 Format	17s
🧩 Workspace deps	15s
🔗 Doc links	7s
🛡 Audit	13s
📏 Bundle size	32s
📦 Pack validation	53s
CI complete	3s
	6m 58s

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