Implement Iterator.remove() on OptimizedTagMap view iterators

[dougqh]

Summary

The view iterators returned by OptimizedTagMap.entrySet().iterator(), keySet().iterator(), values().iterator(), and the TagMap.iterator() overload all extend IteratorBase, which inherited the default Iterator.remove() that throws UnsupportedOperationException. As a result, AbstractSet/AbstractCollection bulk operations that rely on iterator-remove (removeAll, removeIf, retainAll) silently failed mid-iteration on optimized maps — a Map contract gap.

This patch tracks the last-returned entry on IteratorBase and removes by tag through OptimizedTagMap.getAndRemove, which enforces the frozen check. The fix is contained entirely in IteratorBase, so all three subclasses (EntryReaderIterator, KeysIterator, ValuesIterator) inherit remove() automatically.

LegacyTagMap is unaffected: its view iterators are inherited from HashMap and already support remove. (Its TagMap-level iterator() does not, but that's a separate divergence — see follow-up work below.)

Test plan

• New tests in TagMapTest:
  • entrySet_removeIf (parameterized over OPTIMIZED + LEGACY)
  • keySet_removeAll (parameterized over OPTIMIZED + LEGACY)
  • values_removeIf (parameterized over OPTIMIZED + LEGACY)
  • optimizedIterator_remove
  • optimizedIterator_remove_acrossBucketCollisions (forces BucketGroup chains, then removes all entries via iterator)
  • optimizedIterator_remove_beforeNext_throws
  • optimizedIterator_remove_twice_throws
  • optimizedIterator_remove_onFrozenMap_throws
• ./gradlew :internal-api:test passes
• ./gradlew :internal-api:spotlessCheck passes
• CI matrix

Follow-up not in scope

• LegacyTagMap.IteratorImpl.remove() is also unsupported (would need to delegate to the wrapped HashMap iterator). Different mechanism, different PR.
• Pre-existing class-init ordering issue: running a single @EnumSource(TagMapType.class) test in isolation (e.g. --tests TagMapTest.numericZeroToBooleanCoercion) fails with NPE in TagMap.<clinit> because TagMapType loads OptimizedTagMapFactory.INSTANCE before TagMap. Reproduces on master without these changes; orthogonal to this PR.

🤖 Generated with Claude Code

[dougqh]

Experiment - based on a Claude review, not quite what I had in mind
I actually wanted to just make a change that forbids removal through Iterators altogether

[dougqh]

Decided this isn't worth pursuing, would be better to just eliminate LegacyTagMap