Demo 5: mixed-precision SDR link demonstrator

[Ravenwater]

Port upstream's SDR link demonstrator to a Python demo, as
demos/05_sdr_link/.

Upstream source: applications/sdr_demo/sdr_demo.cpp (1591 lines, upstream
#104) and the six-page guide under docs-site/src/content/docs/sdr/
(upstream #105).

Why

demos/01_audio_dynamics through demos/04_sdr_ddc_cascade established the
pattern: a README that states the engineering question, a design.py that
produces coefficients, a simulate.py that measures, and an emitter that
writes something a downstream toolchain can consume. Demo 4 already covers the
receiver front end (digital down-conversion). This demo covers the other
half — modulation, shaping, synchronization, demodulation — and is where the
mixed-precision story actually pays off, because it can report EVM and BER
per arithmetic config rather than a filter response.

Scope

• demos/05_sdr_link/README.md — the engineering question and the measured
  answer, in the voice of the existing four.
• design.py — constellation + RRC pulse pair for a chosen modulation.
• simulate.py — full link: map -> shape -> AWGN -> matched filter -> AGC ->
  timing recovery -> carrier recovery -> demap; report EVM, MER, BER against
  theoretical_ber_awgn.
• A precision sweep across the dtype table using the analyze_blocks binding,
  reporting where each block breaks.
• An emitter consistent with the other demos (emit_c_header.py or a coeff
  emitter), for the RRC taps.

Notes

• Do not restate the old 0.6.0 Phase 2: expose CoeffScalar/T in window and FIR design bindings #85 premise. Upstream's own sweep measured the
  opposite of what the epic assumed — see the precision-analysis issue in this
  epic and upstream's sdr/precision.md. Report what this demo measures.
• Tests go in tests/test_demos.py, which already has the
  function-scoped _demo_modules contextmanager that purges and restores
  module names — demos share design.py/simulate.py basenames and collided
  in sys.modules before that helper existed. Use it.
• Keep the runtime sane. The full precision sweep at 2000 symbols across 18
  configs and 5 blocks is not a unit test; gate the expensive path behind a
  __main__ guard and test the cheap one.

Depends on

All binding issues in the epic. This is the last one.

Deliverables

• demos/05_sdr_link/ with README, design, simulate, emitter.
• Coverage in tests/test_demos.py using _demo_modules.
• README module/demo table updated.

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Part of the SDR binding epic #130.