fix: preserve structural sharing for no-op array-methods calls

[maximilliangrand]

Problem

With enableArrayMethods() enabled, mutating array methods that change nothing still produce a new state reference, breaking Immer's structural-sharing guarantee that a change-free producer returns the base.

enableArrayMethods()
const base = {list: [1, 2, 3]}
const next = produce(base, d => d.list.push(...[])) // empty spread
next === base // false  (expected: true)

Same for splice(i, 0), and pop()/shift() on an empty array. The common real-world trigger is draft.list.push(...incoming) when incoming happens to be empty, which silently invalidates memoization and forces re-renders. Native Immer (plugin disabled) returns the base in all these cases.

Cause

The plugin routes every mutating method through executeArrayMethod, which calls markChanged unconditionally, bypassing the no-op detection that core's set trap performs.

Fix

Detect these no-ops in O(1) and skip marking the array changed, so the producer returns the base and emits no patches, matching core behavior. Real mutations and their patches are unchanged.

sort()/reverse() are left untouched: they are genuine reorder operations where cheap no-op detection isn't possible.

Verification

• New tests run in both plugin-enabled and native modes; they fail on baseline (source reverted) and pass with the fix.
• Full src suite and build-config suite pass; prettier --check clean.