fix(flutter): prevent ConcurrentModificationError in SignalsMixin

[DavidPluxia]

Summary

Fixes #431 - ConcurrentModificationError when using SignalsMixin.

Problem

HashMap.values returns a lazy Iterable, not a snapshot. During iteration in _setup(), accessing s.target.value can trigger bindSignal() → _watch(), which mutates _signals while the iterator is active.

Similarly, clearSignalsAndEffects() iterates lazily over _signals and _effects while dispose()/cleanup calls can mutate these collections.

Solution

Convert lazy iterables to List snapshots before iteration:

// Before (lazy - unsafe)
for (final s in _signals.values.where((e) => e.local != null)) {

// After (snapshot - safe)
final signals = _signals.values.where((e) => e.local != null).toList();
for (final s in signals) {

Changes

1. _setup(): Snapshot _signals.values before iterating
2. clearSignalsAndEffects(): Snapshot both _signals.values and _effects before iterating

Minimal, surgical fix with no behavioral changes.

[rodydavis]

LGTM ✅