feat: add validated layout engine

[ddebowczyk]

Summary

• replace path-based reducer internals with a pure, stable-ID command engine
• add strict state validation, detached inspection, and version-2 snapshots with split identities
• route React interactions through the same commands and update the demo and README

This is stacked on #32. Review the commit after 553f577 for this step.

Verification

• npm run build:demo
• npm run build:lib
• NODE_OPTIONS="--localstorage-file=/tmp/react-layman-vitest-localstorage" npm test
• npm run lint
• markdownlint README.md
• git diff --check