Support array defaults without ambiguous sentinel comparisons

[xiehuanyi]

NumPy arrays cannot be used as get/pluck defaults, reduceby initial vectors, or outer join defaults: comparing them directly with the sentinel string produces an array and raises the ambiguous-truth-value error. These are useful defaults when looking up or aggregating vector data.

Restrict the sentinel equality check to strings and UserString values. Keep the existing behavior for the mangled __no__default__ string, including copies, while returning non-string defaults without invoking their equality operator. Add regressions for each affected API and UserString compatibility controls.

This preserves the compatibility behavior discussed in #625. Related #643 addresses accumulate; the changed functions here do not overlap it.

Validation: four NumPy regressions fail on unchanged master; the complete suite passes after the fix (198 tests with NumPy enabled). Pyflakes and line-length checks pass. NumPy is an optional test dependency, not a runtime dependency.

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