Migrate sine plugin to the C++ wrapper API (rename sine.c to sine.cpp)

[kaoskorobase]

Context

plugins/sine.c is implemented against the raw C API (Methcla_SynthDef with C function pointers). plugins/README.md declares the C++ wrapper API "preferred". This issue migrates sine to the wrapper and renames the file to .cpp.

Outcome: sine uses the wrapper API; the last .c source under plugins/ is gone.

Scope

Delete plugins/sine.c and create plugins/sine.cpp as a full rewrite using StaticSynthDef. Behavioural preservation:

• Phase carried across blocks.
• freqToPhaseInc = 2π / sampleRate.
• sinf semantics.

Key shape:

class Sine
{
    float* m_ports[SinePorts::numPorts()];
    double m_phase;
    double m_freqToPhaseInc;
public:
    Sine(const World<Sine>& world, const Methcla_SynthDef*, const NoOptions&)
    : m_phase(0.)
    , m_freqToPhaseInc(2. * kPi / world.sampleRate())
    {}
    // connect, process as usual
};

StaticSynthDef<Sine, NoOptions, SinePorts> kSineDef;

Files

• plugins/sine.c — delete
• plugins/sine.cpp — new (rewrite using wrapper)
• plugins/CMakeLists.txt — line 23: replace sine.c with sine.cpp in the foreach. The if(source MATCHES ".*\\.c$") block on line 14 self-disables once no .c source remains.
• plugins/README.md — line 9: sine.c → sine.cpp; update the "Implementation styles" table row to move sine under the wrapper category.
• CHANGELOG.md — add a ### Changed entry under ## [Unreleased] referencing this issue (mention the file rename).

Verification

pre-commit run --all-files
cmake --preset debug
cmake --build build/debug
ctest --test-dir build/debug --output-on-failure

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