fix: make moveTab validate before committing source removal

[ddebowczyk]

Summary

• Validate tab source, destination, and placement before a source mutation.
• Move the stored tab object, not an action payload that reuses its ID.
• Adjust a tree destination only when removing the source tab collapses its window; preserve an explicit self-edge split and make self-center moves no-ops.
• Keep the legacy external [-1] source at the public boundary but represent it explicitly in the reducer.

Validation

• npm test -- --run tests/reducer.test.ts (42 passed)
• NODE_OPTIONS=--localstorage-file=/tmp/react-layman-pr02-localstorage.json npm test (79 passed)
• npx --no-install tsc --noEmit
• npm run lint
• npm run build:lib

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