Oscillator fallback audit: several engines falling back to default JS voice

[ford442]

Problem

Some oscillator engines are not initializing correctly and silently fall back to the default JavaScript voice instead of their intended engine.

Current Status

Engine	Status	Notes
JS (Native)	✅ Pass	Works as expected
PCM 303	✅ Pass	Works as expected
Open303	❌ Fail	Falls back to default JS voice
JC303	❌ Fail	Falls back to default JS voice
Rust	❌ Fail	Falls back to default JS voice
Prophecy	❌ Fail	Falls back to default JS voice
Pyodide	❌ Fail	Falls back to default JS voice
WebGPU	❌ Fail	Falls back to default JS voice

Checklist

• Open303 – diagnose why engine fails to initialize and falls back to JS voice
• JC303 – diagnose why engine fails to initialize and falls back to JS voice
• Rust – diagnose why engine fails to initialize and falls back to JS voice
• Prophecy – diagnose why engine fails to initialize and falls back to JS voice
• Pyodide – diagnose why engine fails to initialize and falls back to JS voice
• WebGPU – diagnose why engine fails to initialize and falls back to JS voice
• Add console warning or error when fallback occurs (if not already present)
• Update tests to cover each oscillator engine initialization path

Context

Observed during runtime testing. The fallback appears to happen silently without surfacing an error to the user. Each engine likely has a different root cause (WASM load failure, worklet registration error, missing binary, etc.).