Replace C engine API with direct C++ API

[kaoskorobase]

Goal

Drop the C engine API layer and expose a direct C++ API to host applications, eliminating the double-wrap between the C++ Engine class and the C++ Environment internals. The Plugin and Soundfile API interfaces keep their C ABI for DSO stability.

Motivation

The C API exists primarily to support language bindings, which are not a use case for this project. It adds an indirection layer (src/Methcla/API.cpp) that converts C++ exceptions to error codes only for the C++ wrappers to rethrow them, and requires opaque struct wrappers around already-C++ objects.

What changes

Headers removed

• include/methcla/engine.h — C engine API (opaque Methcla_Engine*, Methcla_AudioDriver*, Methcla_EngineOptions*)
• include/methcla/common.h — C error-handling machinery (Methcla_Error, Methcla_ReturnCode, predicates)
• include/methcla/log.h — Methcla_LogHandler (replaced by std::function already in engine.hpp)
• include/methcla/types.h — C enums, replaced by C++ enum class in engine/types.hpp
• include/methcla/plugin.h — moves and splits (see below)
• include/methcla/file.h — moves and splits (see below)
• include/methcla/plugin.hpp, file.hpp, log.hpp — moved into plugin/ directory

New header layout

include/methcla/
  engine/
    engine.hpp        ← C++ Engine class; holds EngineImpl via pimpl (no Methcla_Engine* wrapper)
    types.hpp         ← C++ enum classes: NodePlacement, BusMappingFlags, NodeDoneFlags
    log.hpp           ← LogStream (was top-level log.hpp)
    detail.hpp
    detail/result.hpp
  plugin/
    common.h          ← shared C types: AudioSample, Time, OSCPacket, LogLevel
    synth.h           ← Methcla_World, Host, SynthDef, Library, port types  (was plugin.h)
    soundfile.h       ← Methcla_SoundFile, SoundFileAPI, SoundFileInfo       (was file.h)
    synth.hpp         ← C++ synth plugin authoring helpers                   (was plugin.hpp)
    soundfile.hpp     ← C++ soundfile wrapper                                (was file.hpp)
  plugins/            ← plugin registration headers, unchanged
    sine.h, disksampler.h, ...

Source removed

• src/Methcla/API.cpp — the entire C adapter layer (opaque struct implementations, METHCLA_API_TRY/CATCH, methcla_engine_* exported functions)

Engine class changes

• Drops Methcla_Engine* member; holds std::unique_ptr<EngineImpl> directly (pimpl)
• EngineOptions/AudioDriverOptions hold native C++ types instead of building Methcla_EngineOptions*
• Exceptions propagate directly; no error-code round-tripping
• Methcla_LogLevel in LogHandler becomes a C++ enum

What stays unchanged

• plugin/synth.h and plugin/soundfile.h — full C ABI preserved (Methcla_Host, Methcla_World, Methcla_SynthDef, Methcla_Library, Methcla_SoundFileAPI)
• methcla_api_host_* / methcla_api_world_* functions in Engine.cpp that implement those tables
• plugins/*.h registration headers (only their #include path updates from <methcla/plugin.h> to <methcla/plugin/synth.h>)
• CMake target structure; src/CMakeLists.txt FILE_SET HEADERS list needs updating to new paths

Out of scope

• Changing the OSC command protocol
• Modifying plugin implementations
• Any behaviour change; this is a structural/API-surface refactor only