[BUGFIX] Reject conditioned/gated configs in the A2 fast-path detector

[rhaist]

Summary

The A2 fast-path detector is_a2_shape validated the layer array exhaustively
but never inspected the top-level condition_dsp key. The generic
WaveNet, given a non-null condition_dsp, builds a nested model and routes the
conditioning signal through it before the layer stack (model.cpp:841,
_process_condition). The A2 fast path has no such stage — it feeds the raw
input as the conditioning signal (a2_fast.cpp, cond[f] = x).

Because the condition DSP carries its own weights (WaveNet::set_weights_
skips them, model.cpp:626), the parent weight stream is byte-identical with or
without it, so the loader's weight-count check cannot catch the mismatch. The
dispatcher (model.cpp:1233, create_config) consults is_a2_shape before
it would ever read condition_dsp, so an A2-shaped config that also carries a
condition_dsp was silently routed to the fast path and produced different
audio than the generic WaveNet — with no error.

This contradicts the A2 fast-path's own stated invariant (PR #251: "checks
every knob against the A2 signature … never silently routes a non-A2 model to
the fast path"), and conditioning a WaveNet with a nested model is a real
feature (#188 — "investigated during A2 development to have a conditioning
WaveNet compute the features that condition the main WaveNet").

Fix

• is_a2_shape rejects any config with a non-null top-level condition_dsp.
• Also rejects the legacy boolean gated: true (pre-gating_mode schema),
  which maps to GATED layers the fast path doesn't implement. Previously this was
  caught only indirectly by a downstream weight-count throw; now the detector
  enforces the boundary in one place.

Both fall back to the generic WaveNet, which handles these configs correctly.

Tests

Adds test_detector_rejects_condition_dsp and test_detector_rejects_legacy_gated,
mirroring the existing test_detector_rejects_* cases. Verified both fail
without the guards (the detector wrongly accepts the config — confirmed
end-to-end with a probe showing is_a2_shape returning true on a conditioned
A2 config) and pass with them. Full run_tests suite passes.

[sdatkinson]

Thanks!