fix: preserve live Map iteration during draft mutations

[Marve10s]

Mutating a draft Map during iteration can visit deleted entries and skip added entries when no earlier operation has created the draft's copy.

const seen = []
produce(new Map([["a", 1], ["b", 2]]), draft => {
  draft.forEach((value, key) => {
    seen.push([key, value])
    if (key === "a") {
      draft.delete("b")
      draft.set("c", 3)
    }
  })
})

Previously seen was [["a", 1], ["b", undefined]]; it now matches native Map behavior: [["a", 1], ["c", 3]].

Prepare the copy before forEach and keys start iterating, so iteration and subsequent mutations use the same backing Map. This also fixes values, entries, and Symbol.iterator, which use keys. Read-only iteration now creates a shallow copy, but an unmodified producer still returns the original state. The regression cases cover all five entry points in the existing configuration matrix.

This restores native live-iteration semantics: repeatedly deleting and re-adding the current key, or continually adding keys during traversal, can keep iteration running just as it would on a plain Map.