Bind sdr::AutomaticGainControl

[Ravenwater]

Bind sw::dsp::sdr::AutomaticGainControl — the level-regulating loop that
belongs upstream of every other synchronization block.

Upstream source: include/sw/dsp/sdr/agc.hpp (closes upstream #100).
Upstream tests: tests/test_sdr_agc.cpp.

Surface

• enum class LevelDetector { magnitude, rms }
• AgcConfig<T> — reference_level, attack_time_s, decay_time_s,
  averaging_time_s, sample_rate_hz, min_gain_db, max_gain_db,
  initial_gain_db, detector
• AutomaticGainControl<StateScalar, SampleScalar> — process(sample),
  block process(vector), reset(), set_gain_db(db), gain_db(),
  invariants_hold()

Binding notes

• First consumer of the scalar-pair dispatch decision from the foundations
  issue. <StateScalar, SampleScalar> here is not incidental: the whole point
  of an AGC study is a narrow sample path against a wide-enough loop state, so
  whatever scheme is chosen must be able to express that combination.
• AgcConfig<T> is templated on the state scalar. Binding one config class per
  arithmetic config would be absurd; bind a single float64-valued config struct
  and narrow the fields on construction of the Impl<T>. The config is
  parameters, not state.
• Times are in seconds against sample_rate_hz, and leaving the rate at 1
  makes them sample counts. Say this in the docstring — it is the kind of
  detail that silently produces a loop 48000x too slow.
• invariants_hold() is a debug predicate. Bind it; it is exactly the sort of
  thing a precision study wants to assert at narrow widths.
• Follow the type-erased IImpl + templated Impl<T> + Py* wrapper pattern
  already used in src/acquisition_bindings.cpp and
  src/multirate_bindings.cpp.
• Complex process() — see the foundations issue for the inbound convention.

Depends on

Foundations issue (both decisions).

Deliverables

• Bindings in src/sdr_bindings.cpp.
• Exported by explicit name in python/mpdsp/__init__.py.
• tests/test_sdr_agc.py — settling behaviour, gain clamping at
  min_gain_db/max_gain_db, and a narrow-dtype run that shows where the
  loop stops converging.
• scripts/build_api_ref.py tables updated, docs/api_reference.md
  regenerated.

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Part of the SDR binding epic #130.