Preserve SortedDict invariants when update input raises

[vitalivo]

This replaces #254, which was accidentally closed and its source fork deleted. The implementation is unchanged; the original discussion and reviews remain linked there.

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Calling update() on an empty SortedDict with a malformed pair or an iterator that raises can leave keys in the underlying dictionary but not in its sorted index. After catching the original exception, len(d) disagrees with iteration and _check() fails.

Materialize iterable input before taking the empty-dictionary fast path, as the nonempty path already does. The direct-dict optimization remains. Tests cover malformed pairs with and without a key function, a failing generator, and nonempty controls; three cases fail on the base revision.

Validation: all 372 tests, doctests, and stress tests pass on Python 3.12. Ruff passes for the changed library file. Repository-wide Ruff reports five existing B905 diagnostics in untouched sortedlist.py; formatting reports an existing adjacent string literal at sorteddict.py:194. No new lint or formatting issues were introduced.