Add support for Faust plugins

[kaoskorobase]

Faust plugin support — v1 design

Add support for Faust plugins via libfaust.

Scope: Desktop only (rtaudio/jack) for v1. iOS unsupported (runtime LLVM JIT is
prohibited on iOS); Android deferred. Gated behind a single CMake option, off where
libfaust is unavailable.

Model: Runtime libfaust. Faust factory ≈ Methcla SynthDef; DSP instance ≈ Synth.
Compile a factory once per .dsp; createDSPInstance / deleteDSPInstance per node.

Compilation: On the NRT worker thread. Blocking the serial NRT queue during a compile
is accepted for v1; RT audio is unaffected since the RT thread only ever sees an
already-built def.

Registration — two paths:

• Startup: scan plugin directories for .dsp, compile + register during
  single-threaded init. No race; cost is LLVM at init and slower startup.
• Runtime: new OSC verb (e.g. /faust/compile) carrying .dsp source/path + target
  URI. NRT compiles; the m_synthDefs insertion is bounced onto the RT/engine command
  queue via sendFromWorker, so the unlocked map is mutated only in RT context.
  Afterwards clients instantiate by URI via /synth/new like any synthdef.

Update semantics: Re-registering a URI replaces the def for future instantiations
only. No live hot-swap of running nodes in v1.

Safe retirement (no RT-thread deallocation of LLVM objects): Factory + heap
Methcla_SynthDef owned by an NRT-side registry — never by a node or a node-droppable
shared_ptr. Replaced defs move to a pending-retirement list with a live-instance
counter. Per-node destroy does deleteDSPInstance only. Each Faust synth, at
end-of-life, fires world_perform_command (RT→NRT; callback allocated from the RT pool)
to decrement the old def's count; NRT calls deleteDSPFactory once it reaches zero.

Existing code: faust/architecture/synth.cpp and src/Methcla/Faust.hpp are AOT-era,
written against the defunct MethclaSynth / MethclaHost C++ API and do not match the
current C Methcla_SynthDef plugin API. Not reusable as-is for the runtime path; remove
or shelve for a possible future AOT/iOS path.

Open items:

• Test-harness integration (tests/plugins + engine_tests.cpp) — TBD.
• Exact /faust/compile OSC arg schema (source string vs. file path, URI supply