perf(spectrum): offload FFT to Rust and render bars via canvas

[a-32x]

Summary

Fixes a severe memory leak and CPU bottleneck in the Spectrum node on WebView2 / Windows.

Prior to this change:

• The UI thread received 8,192 raw PCM float samples over IPC (audio://scope) at ~25-30 Hz. In Tauri on Windows, each emit translates to an inter-process COM ExecuteScript call with JSON serialisation, which flooded V8 heap memory up to 5.15 GB and pinned CPU at ~13%.
• Spectrum FFT analysis (4096-point Hann) ran synchronously on the JavaScript main thread.
• Rendering was performed by re-creating 80 dynamic SVG <rect> nodes on every animation frame, triggering continuous DOM reconciliation overhead.

Changes

1. Rust FFT Calculation: Added realfft dependency. Moved 4096-point Hann FFT and 80-bin logarithmic power aggregation into the background meter thread (meter-tick). It now emits a lightweight payload of 80 dB floats over audio://spectrum.
2. Canvas Rendering: Switched the spectrum bar visualizer in spectrum.svelte to a flat HTML5 2D Canvas (fillRect), keeping axes and labels as crisp vector SVG overlays.
3. Event Teardown: Updated tauriListen to return an explicit cleanup function and guard against listener leaks during component unmount races.

Results

• WebView2 renderer process: dropped from 5,154 MB to 162 MB (~97% reduction on the leaking renderer process).
• Total application memory: reduced by ~10x (from ~5.2 GB down to ~300–500 MB across all processes).
• CPU usage: reduced from ~13% to baseline (<1%).
• Framerate: smooth 60 FPS without memory accumulation or garbage collection pauses.

Dependency Note

Added realfft = "3.4" to src-tauri/Cargo.toml with opt-level = 3 in dev profile. RealFFT was chosen because it operates purely on real-valued inputs with half the memory and computation overhead compared to general complex FFTs.

Verification

• Developed on: Windows 11 (x64)
• Tested on: Windows 11 — continuously monitored RAM/CPU over extended playback, verified canvas bar responsiveness across sine, sweep, and noise signals.
• Not tested: macOS, Linux (Rust pipeline and Canvas rendering are platform-agnostic, but requires maintainer verification).

[Horuse]

Thanks for contributing to the project! This is a pretty serious issue that I hadn't noticed, and I appreciate you fixing it. A few things before merging:

1. The canvas is blurry on Retina because it's sized in CSS pixels. It needs devicePixelRatio scaling; waveform_scope.svelte (~line 790) already does this.
2. The spectrum story in Storybook is empty now: src/stories/harness/signal.ts still sends raw audio://scope data for spectrum. It should send bars on audio://spectrum.