feat(audio): add C++ realtime DSP core for v0.2.0

[TheZupZup]

Why C++ here

Linthra already has ReplayGain-style volume normalization in Dart/just_audio, but future EQ/limiting/loudness work needs a realtime-native audio path. This creates a genuine C++ contribution area around DSP rather than adding C++ only for repository language stats.

What ships in this PR

• new native/linthra_audio C++17 library
• allocation-free / lock-free mono-stereo processing callback
• preamp
• up to 8 parametric peaking-EQ bands
• stereo-linked peak limiter with immediate attack and smooth release
• transparent bypass coverage
• stable C ABI for the future Android/JNI or Dart FFI binding
• unit tests for bypass, preamp, EQ, limiter stereo linking, and the C ABI
• 60-second / 48 kHz stereo / 5-band EQ + limiter realtime regression benchmark
• dedicated CMake CI
• contributor README describing the realtime constraints

Deliberate safety boundary

This does not silently replace just_audio or alter production playback yet. Inserting PCM processing into the current Android/platform decoder path is a separate integration concern and needs its own review for buffering, audio focus, F-Droid reproducibility, and transparent bypass.

The DSP math and realtime guarantees are therefore established first; the mobile audio-processor bridge can build on a tested core instead of landing everything at once.

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