fix: stop iteration from revisiting entries moved by get()

[chuanghiduoc]

Iterating with forEach/keys()/entries()/values() while calling get() on the visited entry moves it to the MRU end via #moveToTail. The index walkers follow the re-linked prev/next pointers and land back on nodes they already yielded, so the loop revisits the same entries forever until something throws:

const c = new LRUCache({ max: 3 })
c.set('a', 1); c.set('b', 2); c.set('c', 3)

for (const k of c.keys()) {
  c.get(k)
}
// visits c,b,c,b,c,b,... forever
// eventually: RangeError: Invalid array length

Walk-through of the two-entry cycle (#indexes, walking tail → head via #prev):

1. visit c (tail), callback get('c') — no-op, already tail. Next: prev[c] = b.
2. visit b, callback get('b') — moveToTail(b) unlinks b from between a and c, relinks a↔c, appends b after c (tail = b). List is now a ↔ c ↔ b.
3. walker does i = prev[b] = c → visits c again, whose get restores the old order… and the walk oscillates between steps 2–3 indefinitely.

Deletes during iteration were already handled by the #isValidIndex check (the entry's index no longer maps back to its key); this covers the move case.

Fix

Track yielded indexes in #indexes / #rindexes and skip them if the walk circles back. Each entry is now visited at most once per iteration, matching Map iteration semantics.

The Set is allocated once per iteration call; for the common case where nothing mutates mid-walk, behavior and results are unchanged.

Tests

test/get-during-iteration.ts asserts that iterating while calling get(k) terminates and visits each key exactly once — fails on master (infinite loop), passes with this change.

Full suite: 18430 total / 18364 pass / 66 fail on this branch vs 18326 / 18258 / 68 on unmodified master. The only failing file in both runs is the pre-existing flaky test/tracing.ts; the remaining delta is subtests of my new test file plus run-to-run noise in the same tracing/fetch tests.