fix(core): guard waterfall next against double invocation

[rekcilyssup]

Closes #42

Problem

ctx.waterfall() passed a single shared next continuation to every
middleware frame. Calling it twice in one frame advanced the shared queue
again and re-invoked the terminal callback, duplicating terminal side
effects.

Fix

In packages/core/src/events.ts, each middleware frame now receives its own
guarded continuation:

• the guard is scoped to a single frame — calling the same next twice
  throws next() should only be called once;
• the chain still passes the current continuation as the last argument to
  every listener and the terminal, preserving the existing contract used by
  internal/update, internal/get, and internal/set;
• valid sync, async, and nested chains are unaffected: frames advance exactly
  once and the terminal runs exactly once.

Tests

Added 3 tests: double next() in one frame throws while the terminal runs
once, reusing a previous frame's next throws, and an async listener calling
next() after await still completes the chain in order.

Verified locally: yarn lint, yarn build, yarn test (170 passed).