Add comprehensive documentation for virtual device emulator ...

- tools/virtual_devices/README.md: full user guide with CLI, curl, Web UI
- docs/virtual-device-emulator.md: architecture docs with end-to-end example
- docs/architecture.md: mention virtual emulator in device section
- CLAUDE.md: add virtual-device-emulator.md to key files table

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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