fix: interpretation-layer bugs (pandas NaN fallback, typename parsing, strided objects, bitset)

[henryiii]

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Part of #1646.

Fixes a batch of verified bugs in the interpretation layer, found during review.

Fixed

1. library.py — pandas all-NaN fallback. _process_array_for_pandas fell off the end returning None when interpretation.awkward_form(...) raised CannotBeAwkward, so library="pd" silently built pandas.Series(None, index=...) (all-NaN) instead of the object array. Now returns the array.
2. identify.py — parse_typename for long long* / unsigned long long*. These branches were dead (shadowed by the scalar long/unsigned long branches, which consume the token) and constructed AsArray({header}, dtype) with the wrong arity. Reordered so the pointer forms are checked first and pass the missing speedbump argument.
3. identify.py — broken assert message. The assert's f-string message was a separate statement (dead code, truncated message) and validation shouldn't be an assert (stripped under -O). Replaced with a raise ValueError(...).
4. objects.py — AsStridedObjects:
   • __init__ deleted from a list while iterating range() of its original length (IndexError with mid-list (None, None) header markers from multiple base classes) and could drop the sole member. Rewritten as a filter that drops interior header markers; all_headers_prepended semantics preserved.
   • basket_array mutated shared self._to_dtype based on a basket's byte offsets — a race / state leak across basket-worker threads. The header dtype is now computed locally, and final_array derives the output dtype from the actual basket arrays.
   • Hoisted the per-basket awkward_form(...) recomputation out of the loop, reused has_any_awkward_types, removed a dead container = {}.
5. strings.py — final_array. output could be referenced unbound for mixed StringArray/awkward baskets; now bound on all paths.
6. containers.py:
   • STLBitSet.__str__ referenced a nonexistent self._values and __len__ returned an ndarray (both TypeError); fixed to use self._numbytes and return an int.
   • AsBitSet now keeps its bit count and renders std::bitset<N> instead of std::bitset<bool>.
   • Simplified always-true KeysView is not None / ValuesView is not None (Py2 leftovers).
7. known_forth/init.py — fallback typename = classname_decode(model.__name__) returned a (classname, version) tuple that could never match the string keys in KNOWN_FORTH_DICT; now takes [0].
8. custom.py — AsBinary (library="np"). The uniformity check numpy.unique(counts[1:] - counts[:-1]).size == 1 failed on single-entry baskets (empty diff → AssertionError) and passed for any arithmetic progression. Changed to numpy.unique(counts).size <= 1 on the per-entry sizes.
9. python.py — expression compilation. A branch named like a function (e.g. sqrt) used in call position was rewritten to get('sqrt'), breaking sqrt(x). Names in func position that exist in the functions table now resolve to the function and emit NameConflictWarning when shadowed by a branch.
10. library.py — perf. _strided_to_awkward now uses data.reshape(-1) (view) instead of data.flatten() (copy) at every recursion level.

Skipped

None — all ten findings held up under verification.

Tests

Regression tests added in tests/test_<N>_*.py (named after the PR number) and the targeted/interpretation-heavy suites pass locally.

[codecov]

Codecov Report

❌ Patch coverage is 87.61905% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.97%. Comparing base (1c06db0) to head (0776537).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/uproot/interpretation/identify.py	50.00%	3 Missing and 3 partials ⚠️
src/uproot/interpretation/objects.py	96.66%	1 Missing and 1 partial ⚠️
src/uproot/interpretation/strings.py	0.00%	2 Missing ⚠️
src/uproot/language/python.py	86.66%	1 Missing and 1 partial ⚠️
src/uproot/containers.py	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/uproot/interpretation/custom.py	85.52% <100.00%> (+7.89%)	⬆️
src/uproot/interpretation/known_forth/__init__.py	100.00% <100.00%> (ø)
src/uproot/interpretation/library.py	63.96% <100.00%> (+0.42%)	⬆️
src/uproot/containers.py	67.04% <91.66%> (+2.71%)	⬆️
src/uproot/interpretation/objects.py	85.56% <96.66%> (+1.60%)	⬆️
src/uproot/interpretation/strings.py	69.76% <0.00%> (-0.66%)	⬇️
src/uproot/language/python.py	72.32% <86.66%> (-0.89%)	⬇️
src/uproot/interpretation/identify.py	68.82% <50.00%> (+0.70%)	⬆️