Migrate patch_cable PatchCable to the C++ wrapper API

[kaoskorobase]

Context

plugins/patch_cable.cpp currently registers two SynthDefs:

• PatchCable — implemented against the raw C API (Methcla_SynthDef with construct/connect/process/destroy function pointers).
• Amplifier — already uses the C++ wrapper API (StaticSynthDef).

plugins/README.md declares the wrapper "preferred". This issue brings PatchCable in line.

Outcome: patch_cable.cpp uses the wrapper for both synths; ~50 lines of raw-C boilerplate dropped.

Scope

Replace the raw-C PatchCable synth (lines 25–92 of plugins/patch_cable.cpp) with a wrapper class alongside the existing Amplifier. Update the loader to call kPatchCableDef(host, METHCLA_PLUGINS_PATCH_CABLE_URI).

typedef NoOptions PatchCableOptions;

class PatchCablePorts
{
public:
    enum Port { kInput, kOutput };
    static constexpr size_t numPorts() { return 2; }
    static Methcla_PortDescriptor descriptor(Port port)
    {
        switch (port)
        {
            case kInput:  return PortDescriptor::audioInput();
            case kOutput: return PortDescriptor::audioOutput();
            default:
                throw std::runtime_error(METHCLA_PLUGINS_PATCH_CABLE_URI
                                         ": Invalid port index");
        }
    }
};

class PatchCable
{
    float* m_ports[PatchCablePorts::numPorts()];
public:
    PatchCable(const World<PatchCable>&, const Methcla_SynthDef*,
               const PatchCableOptions&) {}
    void connect(PatchCablePorts::Port port, void* data)
    {
        m_ports[port] = static_cast<float*>(data);
    }
    void process(const World<PatchCable>&, size_t numFrames)
    {
        const float* src = m_ports[PatchCablePorts::kInput];
        float*       dst = m_ports[PatchCablePorts::kOutput];
        std::copy(src, src + numFrames, dst);
    }
};

StaticSynthDef<PatchCable, PatchCableOptions, PatchCablePorts> kPatchCableDef;

Files

• plugins/patch_cable.cpp — replace the raw-C PatchCable synth
• plugins/README.md — update the "Implementation styles" table row for patch_cable.cpp (already lists wrapper for Amplifier; both synths now use wrapper)
• CHANGELOG.md — add a ### Changed entry under ## [Unreleased] referencing this issue

Verification

pre-commit run --all-files
cmake --preset debug         # or whatever preset name is in CMakePresets.json
cmake --build build/debug
ctest --test-dir build/debug --output-on-failure

Confirm methcla_plugin_patch_cable builds. Existence + load is sufficient — no nontrivial state to behaviourally diff.