feat: add pick command for reduce

[henryiii]

This is a potential implementation of a pick command.

This is a test run of Claude Fable 5, which was released today.

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Closes #275.

This adds a pick command to algorithm::reduce, which selects an arbitrary subset of bins by index from an axis which is not ordered, like the category axis.

auto h = make_histogram(axis::category<std::string>({"red", "green", "blue"}));
// ...
auto h2 = algorithm::reduce(h, pick({2, 0})); // axis is now {"blue", "red"}

Design

• pick(iaxis, indices) / pick(indices) follow the same positional/explicit-index convention as the other reduce commands. Indices must be unique and non-empty (std::invalid_argument otherwise); out-of-range indices are rejected inside reduce.
• Unlike slice, the picked bins do not have to be adjacent, and the order of the indices determines the bin order of the new axis, so pick can also reorder an unordered axis.
• Counts in bins which are not picked are added to the overflow bin if present, otherwise discarded — consistent with how slice already treats unordered axes.
• pick cannot be fused with any other command for the same axis.
• As suggested in Add pick command for reduce #275, this introduces a new kind of reduce constructor: A(const A& src, const std::vector<index_type>& indices). A new public trait axis::traits::is_pickable detects it, mirroring is_reducible, so user-defined axes can opt in. axis::category implements it; ordered axes (regular, variable, integer, circular) throw, since picking a non-adjacent subset from them would require changing the axis type, which reduce does not support yet (see discussion in Add pick command for reduce #275 about a future discontinuous axis / type-changing reduce).

Testing

• New tests in algorithm_reduce_test.cpp: pick with/without overflow bin, reordering, 2d correlation preservation, static and dynamic histograms (through axis::variant), the iterable overload, and all error paths (empty, duplicate, out-of-range indices, non-pickable axis, fusion conflicts). New code paths are fully exercised for the coverage gate.
• is_pickable true/false cases in axis_traits_test.cpp.
• Docs: changelog, guide reduction section, Axis concept (new special constructor), and a runnable example in guide_histogram_reduction.cpp.

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