The process adapters (process/ and process_bus/, per-channel and poly) already
had storage to point missing channels at silent scratch buffers, but it was
gated behind a commented-out AVND_ENABLE_SAFE_BUFFER_STORAGE, so by default a
host supplying fewer channels than the object declares left bus.channel = nullptr
-- any effect that dereferences its channel then crashes. That null path is a
core-invariant violation: an effect must always receive valid, zero-filled
channel buffers.

Enable the safe buffer storage by default (opt out with
-DAVND_ENABLE_SAFE_BUFFER_STORAGE=0). process_bus/base.hpp includes
process/base.hpp, so this one definition covers every backend.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_011s7huWR2wFsLFiMJPjx1z2

Debugged with cdb against clap-validator and the Steinberg validator;
seven distinct defects, taking the examples from 53/85 fully passing
clap-validator runs to 84/85:

- clap: 32/64-bit sample format is a per-process() host decision
  (SUPPORTS_64BITS is capability, not contract): read whichever of
  data32/data64 the host filled; the adapters convert. Was: double
  processors dereferenced a null data64 on every float host.
- clap: allocate float AND double conversion storage in start() (the
  float->double path had a null destination — the second crash class).
- clap: null channel rows inside host buses are pointed at binding-owned
  zero/trash scratch (sized at activate) — no adapter dereferences null.
- clap: note ports declared no dialects (preferred_dialect = 0 is
  invalid): supported = MIDI|CLAP, preferred MIDI.
- clap: multi-bus plug-ins reported the plug-in-wide channel total and
  CLAP_PORT_STEREO on every port: each port now reports its own count.
- clap: values-list controls (combos) reported min=max=0: 0..N-1 stepped.
- clap+vst3: channel-port effects (mono .channel members — analyzers like
  essentia Entropy) used the *bus* introspection in bus_info, reporting
  zero buses (#154); use the channel introspection. The vst3 no-match
  default also derives its buses from the wrappers' channel counts now.
- all three bindings: sample-accurate control ports get their per-buffer
  storage reserved at activate and cleared around process() — effects
  read the `values` arrays which were dangling (heap-garbage crash in
  SampleAccurateControls).
- clap: host-callback members are never left as empty std::functions
  (bad_function_call terminates under -fno-exceptions): workers run
  inline, variable-channel requests are accepted as no-ops until CLAP
  port renegotiation is implemented.

107/107 tests pass. Remaining: avnd_test_midi_out (3 clap failures),
avnd_test_audio_variable.vst3 (validator crash), param-only test objects
under vst3 (no buses by design).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_014Zm2k4dsLp2jMa47zyLcSZ