Bind sdr::Constellation — QAM/PSK mapper and demapper

[Ravenwater]

Bind sw::dsp::sdr::Constellation and its free helpers — the symbol mapper
and demapper at the head of the SDR chain.

Upstream source: include/sw/dsp/sdr/constellation.hpp (closes upstream #95).
Upstream tests: tests/test_sdr_constellation.cpp.

Surface

enum class Modulation { bpsk, qpsk, psk8, qam16, qam64, qam256 } plus
to_string.

Free functions:

• bits_per_symbol(Modulation) -> 1, 2, 3, 4, 6, 8
• modulation_order(Modulation) -> 2^bits
• is_square_qam(Modulation)
• binary_to_gray(std::size_t), gray_to_binary(std::size_t, nbits)

Constellation<T>:

• symbol(index), index_of(bits), bits_of(index, out)
• map(std::span<const std::uint8_t> bits) -> symbol
• demap_hard(symbol) -> index, demap_hard_bits(symbol, out)
• demap_llr(symbol, noise_variance, out) — exact
• demap_llr_maxlog(symbol, noise_variance, out) — max-log approximation
• average_power(), order()

Binding notes

• Modulation should be a proper nb::enum_, not a string key — it is a
  signal-structure parameter, not an arithmetic config, and it recurs in
  metrics.hpp (theoretical_ber_awgn) and analysis/sdr_precision.hpp
  (SdrLinkConfig). Bind it once here and reuse.
• std::span<const std::uint8_t> bit vectors map to uint8 ndarrays. This is
  the first non-float64 array type at the boundary in this repo — the
  np_f64_ro typedefs in src/_binding_helpers.hpp need a uint8
  counterpart with the same c_contig copy-on-noncontiguous behaviour.
• The LLR outputs are written into caller-provided std::span<T>. Bindings
  should allocate and return instead, via make_f64_array — and per
  src/BINDING_PATTERNS.md, any property returning a capsule-owned ndarray
  needs explicit nb::rv_policy::take_ownership.
• Add block-level convenience wrappers (map/demap a whole symbol vector) rather
  than forcing per-symbol Python loops. A 2000-symbol link at one Python call
  per symbol is not usable.
• Constellation<T> is single-scalar, so it dispatches on plain dtype=. It
  does not need the scalar-pair scheme.

Depends on

Foundations issue (complex marshalling — symbol_type is complex).

Deliverables

• src/sdr_bindings.cpp created; forward-decl + call in src/bindings.cpp;
  source listed in nanobind_add_module(_core ...) in CMakeLists.txt.
• Names exported by explicit name in python/mpdsp/__init__.py.
• tests/test_sdr_constellation.py, mirroring the binding file 1:1.
• Entries added to scripts/build_api_ref.py CATEGORIES/CLASSES and
  docs/api_reference.md regenerated (never hand-edited).

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Part of the SDR binding epic #130.