fix: createChannelFromSelector emits a transition twice when the worker dispatches synchronously (baseline recorded after emit)

[panghy]

Sibling of #20 (same function, independent bug; reproduces on stock 0.2.7 with or without the #20 buffer fix).

Summary

createChannelFromSelector (src/utils/sagas/selector-channel-effects.ts) updates its comparison baseline after calling the emitter:

const payload = cachedSelector(reduxStore.getState(), ...args);
if (shallowEqual(prevValue, payload)) return;
emitter({ payload, prevPayload: prevValue });
prevValue = payload;   // too late — see below

emitter() is synchronous: with a taker attached it resumes the take*FromSelector loop, which forks the worker, and the worker's queued put is flushed by the redux-saga scheduler before emitter() returns. If that dispatch is a reducer no-op, the store still notifies subscribers, emitCurrentValue re-enters while prevValue is still the old value, shallowEqual(old, payload) is false again, and the same transition is emitted a second time (payload B, prevPayload A, twice) for a single store update.

If the worker dispatches a real state change instead, the nested emission carries the wrong prevPayload (A → C instead of B → C).

Repro

Real Redux createStore + saga middleware, takeEveryFromSelector(selectValue, worker):

1. Initial state { value: 'A' }, selector state => state.value.
2. Worker records [prevPayload, payload], then yield put({ type: 'noop' }) (reducer returns the same state).
3. store.dispatch(set('B')).

Observed: worker called twice with ['A', 'B']. Expected: once.

Variant: worker puts set('C') when it sees B → observed [['A','B'], ['A','C']], expected [['A','B'], ['B','C']].

Suggested fix

Record the baseline before emitting, keeping prevPayload semantics:

const prevPayload = prevValue;
prevValue = payload;
emitter({ payload, prevPayload });

The selector still throws before the assignment, so reportRuntimeError handling and the retained old baseline on selector failure are unchanged.

Downstream tracking: intent-hq/intent#5040 (interim pnpm patch in cloudlands-fe, alongside the #20 patch).

[panghy]

Interim downstream fix (pnpm patch + regression tests): intent-hq/cloudlands-fe#2472