feat: replace layout model with stable snapshots

[ddebowczyk]

Summary

• replace tab and window state with stable ID-based values and explicit selection
• validate strict schema-versioned snapshots for persistence and host integrations
• split reducer, tree, path, floating-window, and drag concerns into focused modules
• reject invalid or self-targeting move commands before they mutate state
• remove TabData, migrations, normalizers, and redundant move payloads

Verification

• npm run build:lib
• NODE_OPTIONS='--localstorage-file=/tmp/react-layman-vitest-localstorage' npm test
• npm run lint
• git diff --check