fix: make moveWindow source-authoritative and atomic

[ddebowczyk]

Summary

• Resolve and validate both move endpoints before mutating the source state.
• Move the stored source window and ignore the deprecated caller-supplied window payload.
• Reject stale tiled paths, malformed or unseeded new floating destinations, unsupported floating edge drops, and self-targeting tree moves.
• Preserve valid tree, floating, and root-edge transfers while adjusting a destination after source removal.

Validation

• npm test -- --run tests/moveWindow.test.ts tests/reducer.test.ts (39 passed)
• NODE_OPTIONS=--localstorage-file=/tmp/react-layman-pr03-localstorage.json npm test (76 passed)
• npx --no-install tsc --noEmit
• npm run lint
• npm run build:lib

Plain npm test has the known Node 26 localStorage setup failure in persistence tests. PR 04 owns that portable test setup.