fix(core): support symbol and prototype-named events

[rekcilyssup]

Closes #50

Problem

• ctx.on() accepts symbols, but emit() / parallel() / serial() / bail()
  threw TypeError: name.startsWith is not a function on dispatch — _resolve()
  narrowed every event name to string and called startsWith() unconditionally.
• String names colliding with Object.prototype properties (e.g. __proto__)
  failed during registration, because _hooks was a plain object whose inherited
  prototype shadowed own keys.
• Disposing the last listener left the (now empty) event bucket behind.

Fix

In packages/core/src/events.ts:

• _hooks is now a null-prototype table, so prototype-named keys are plain own
  properties.
• _resolve() only applies the internal/ prefix check to string names; symbols
  pass through to the hook table unchanged.
• register() / unregister() now track the event key, and unregister() deletes
  the bucket when the last listener is disposed.

Tests

Added 5 tests: symbol dispatch via emit and parallel, __proto__ registration
and dispatch, and bucket release after disposing the last listener.

Verified locally: yarn lint, yarn build, yarn test (167 passed).