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feat!: route subscriber errors to a configurable onError (default console.error) (#83)

* feat!: route subscriber errors to a configurable onError (default console.error)

A throwing subscriber previously re-threw asynchronously via setTimeout, which
terminates a Node host process when no uncaughtException handler is registered
(fine in browsers — asymmetric and dangerous for SSR/edge/test runners). The
security audit rated this the top runtime risk.

- createBus/createPubSub accept `onError?: (error: unknown) => void`, defaulting
  to console.error. Delivery to the other subscribers always continues.
- New exported `ErrorHandler` type.
- Tests: onError invoked with the error and never re-throws (mutation of intent
  vs the old re-throw test); default-to-console.error; isolation preserved.
- e2e smoke (CJS + ESM): a throwing subscriber does not crash the process.

BREAKING CHANGE: subscriber errors no longer re-throw asynchronously; they go to
onError (default console.error). Pass `createPubSub({ onError })` to customize,
or `onError: (e) => { setTimeout(() => { throw e }) }` to restore the old behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(pubsub): guard against a throwing onError; strengthen error tests

Adversarial review found that a user onError that itself throws would propagate
out of the delivery setTimeout and re-introduce the uncaught-exception crash
(and abort delivery to remaining subscribers). Wrap the onError call in its own
try/catch (EventEmitter-style) so it can never crash the host.

Tests from review:
- a throwing onError neither crashes delivery nor stops other subscribers
- default sink asserts console.error is called WITH the thrown error
- contract spec asserts the error reached the default sink (errSpy called once)
- e2e (CJS+ESM): a throwing subscriber on the PubSub singleton does not crash

107 tests, 100% coverage, e2e 10/10.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthr... (continued)

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