Handle iterable operands consistently in set operations

[mikamikasuki]

IntervalTree documents iterable operands for its set operations, but intersection(iter(tree)) raises TypeError, and membership checks in difference and intersection_update can consume an iterator and return incorrect results. difference_update(tree) also raises RuntimeError when the operand is the tree itself.

Convert non-set operands once before membership queries, preserving existing set operands without copying them. Snapshot the operand for difference_update before mutation so aliases are safe. Regression tests cover iterators, generators, reusable containers, self-difference updates, and set operands whose iteration is counted.

Validation: all 132 tests and doctests pass on CPython 3.12.15 and 3.14.7. Seven regression cases fail before the fix. Separately, 32,000 comparisons against Python set operations pass on CPython 3.9 and 3.14, including mutable interval payloads and self operands; allocation checks confirm that set, frozenset, and IntervalTree operands are not copied by the three query methods. The configured fatal-error flake8 check passes.

Implementation and validation were produced with an AI coding agent.