feat(atoms): bundle custom events with change/mutate through MappedSignals

[bowheart]

Summary

• Custom events dispatched alongside signal.set() or signal.mutate() now stay bundled with the change (and mutate) event when propagating through MappedSignal layers
• Previously, listeners at each MappedSignal layer received two separate callbacks — one for custom events, one for change/mutate. Now they receive a single unified event map
• Standalone .send() events (no state change) are still forwarded immediately as before
• Works recursively through any depth of signal/atom/MappedSignal composition

This also fixes a state tearing bug where mapped signals passed events dispatched along with .set and .mutate calls of inner signals to downstream listeners too early. Those listeners then ran before the inner update had fully propagated through the signal graph. That means those listeners would have to access the inner signal directly to read its current state since the outer signals that the listener is registered on don't have it yet. Deferring the passing along of those inner events in mapped signals prevents this bug completely.

Test plan

• 34 new tests covering single-signal wrapping, map-mode, 2/3/5-deep nesting, mixed-mode nesting, multiple custom events, payload integrity, no-double-notification, and edge cases
• All 142 pre-existing related tests pass (mapped-signal-events, mapped-signals, events, signals, proxies)
• Full test suite passes (610/610 tests across 65 suites)

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