Bind sdr link metrics — EVM, MER, BER, I/Q imbalance

[Ravenwater]

Bind the SDR link-quality metrics — the measurement layer the whole
mixed-precision SDR study reports against.

Upstream source: include/sw/dsp/sdr/metrics.hpp (closes upstream #97).
Upstream tests: tests/test_sdr_metrics.cpp.

Surface

Result structs: EvmResult, BerResult, IqImbalance.

Free functions:

• evm(reference, measured) -> EvmResult
• mer_db(reference, measured) -> double
• ber(transmitted, received) -> BerResult
• theoretical_ber_awgn(Modulation, ebn0_db) -> double
• iq_imbalance(reference, measured) -> IqImbalance
• q_function(x) -> double

Binding notes

• The templated ones are on typename Complex, not DspField — they are
  measurement code and run in double. No dtype= on these. That is the
  same reasoning that makes signal generators reference-precision: a metric
  that quantized itself would confound the thing it is measuring.
• evm, mer_db and iq_imbalance take pairs of complex spans. This is the
  heaviest consumer of the inbound complex-array convention — whatever the
  foundations issue decides, these four signatures are where it shows.
• ber takes std::span<const std::uint8_t> bit vectors; same uint8 ndarray
  typedef needed by the constellation issue.
• theoretical_ber_awgn needs the Modulation enum bound by the constellation
  issue. If metrics lands first, bind the enum here instead — but bind it
  exactly once.
• Result structs get def_ro properties. They are plain scalar aggregates, so
  the take_ownership rule does not bite here.
• require_paired throws on length mismatch upstream; confirm the exception
  surfaces as a Python ValueError and test it.

Depends on

Foundations issue (complex array marshalling); Modulation from the
constellation issue.

Deliverables

• Bindings in src/sdr_bindings.cpp.
• Exported by explicit name in python/mpdsp/__init__.py.
• tests/test_sdr_metrics.py, including a check that measured BER tracks
  theoretical_ber_awgn within a tolerance that scales with the error
  count (upstream hit exactly this flake — see 787cc37).
• scripts/build_api_ref.py tables updated, docs/api_reference.md
  regenerated.

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Part of the SDR binding epic #130.