refactor: code quality improvements

[Detair]

Summary

Four code quality improvements for the v1.0 roadmap:

1. Graceful Error Handling in Audio Callback

• Replaced assert_eq! in process_frame() with a guard clause that outputs silence on channel mismatch
• Prevents DAW crashes when VST3/CLAP/LV2 hosts pass unexpected channel counts

2. Core Unit Tests (21 new tests)

• NoiseFloorTracker (5 tests): initial floor, convergence, near-zero rejection, downward tracking, ring buffer wrap
• ThreeBandEq (3 tests): flat EQ identity, construction, gain updates
• LookaheadLimiter (4 tests): quiet boost, loud attenuation, no clipping, empty frames
• VoidProcessor (6 tests): construction, silence to silence, bypass, gate closing, channel mismatch, process_updates
• FrameAdapter (3 tests): roundtrip, mono duplication, partial frame buffering

3. Ring Buffer Adapter Extraction

• New FrameAdapter struct in voidmic_core encapsulates the interleave, ring buffer, process, deinterleave pattern
• Refactored both plugin/lib.rs and lv2/lib.rs to use it (around 80 lines of duplicated code removed)
• Removed direct ringbuf dependency from plugin and LV2 crates

4. GUI Module Split

• Split gui.rs (1,424 lines) into 8 focused modules under gui/:
  • mod.rs — module declarations
  • app.rs — VoidMicApp struct, constructor, eframe::App impl
  • engine.rs — start/stop/toggle engine
  • tray.rs — tray icon constants and loading
  • devices.rs — device selectors, get_devices, virtual cable installer
  • controls.rs — presets, threshold/suppression sliders
  • advanced.rs — EQ, VAD, AGC, bypass, spectrum, jitter
  • wizard.rs — first-run wizard (5 steps)

Also fixes

• Pre-existing clippy warnings (too_many_arguments, collapsible_else_if)

Verification

• cargo build --workspace — clean
• cargo clippy --workspace -- -D warnings — 0 warnings
• cargo test --workspace — 37 tests pass (16 app + 21 core)

Stats

19 files changed, 1978 insertions, 1559 deletions