fix(utils): capture entry serial in List.push disposer - #71

Problem

@cordisjs/utils's List.push() registers an effect whose disposer closes over this.sn — the mutable field — instead of the serial number of the entry it created:

push(value: T) {
  this.ctx.effect(() => {
    this.inner.set(++this.sn, value)
    return () => this.inner.delete(this.sn) // reads the *current* sn at dispose time
  }, `${this.trace}.push()`)
}

Once a second entry is pushed, this.sn has advanced, so disposing the first entry removes the most recent one rather than its own.

This is observable on fiber reload: unloading runs the disposers in reverse order, so the first-pushed entry is never removed and gets re-pushed on the next load. list.length grows and iterating yields stale duplicates:

const ctx = new Context()
let list: List<string>
const fiber = ctx.plugin((sub) => {
  list ??= new List(sub, 'test')
  list.push('a')
  list.push('b')
})
await fiber
console.log([...list], list.length) // ['a','b'] 2

fiber.update({})
await fiber
console.log([...list], list.length) // ['a','a','b'] 3  ← stale entry leaked

Fix

Capture the serial number at push time (matching the core DisposableList implementation in packages/core/src/utils.ts), so each disposer removes exactly the entry it created:

push(value: T) {
  this.ctx.effect(() => {
    const sn = ++this.sn
    this.inner.set(sn, value)
    return () => this.inner.delete(sn)
  }, `${this.trace}.push()`)
}

Tests

Added packages/utils/tests/index.spec.ts (the @cordisjs/utils package previously had no tests) covering:

• basic push + iteration/length
• no stale-entry leak across fiber reloads

Verified the regression test fails on main (['a','a','b']) and passes with the fix. Full suite remains green (165 tests, 20 files).