fw-01 step 02 — Feel the bugs, then add the guards

Goal

Each stage-4 test corresponds to a real defence in Redux. Reproduce the bug first; only then write the guard.

Dispatch inside a reducer. Reproduce it. Observe that nothing throws and the result depends on ordering. Then add isDispatching.
Unsubscribe during notification. Build the version that iterates the live array with an index and splices on unsubscribe. Watch a listener get skipped. Then snapshot the list — and note that Redux's documented semantic is that a listener unsubscribed mid-dispatch is still called that time.
Mutating reducer + memoised selector. Reproduce a stale UI with no error. This is stage 5, which passes when the bug is present: read it, do not fix it in the store.
Middleware re-entrancy. Dispatch synchronously from within middleware; find the loop.
Re-run the spec; 15/15.