Solidify playback API, rendering pipeline, and coverage

[heucuva]

Added

• Explicit Advance/Render API alongside compatibility Tick; sequencing and render separation tests.
• Mixer read-only accessor (MixerConfig) while keeping legacy pointer for compatibility.
• Focused tests across player/machine lifecycle, feature parsing, playback contracts, period/frequency math, mixers/panning, filters/DSP, oscillators, instruments, song helpers, tracing/logging, voice components (including envelopes, autovibrato, sampler behaviors), format loaders (IT/S3M/XM/MOD), and utility helpers.
• Documentation notes covering sampler concurrency expectations and mixer ownership/callback behavior.

Fixed or Changed

• Tick now enforces a non-nil sampler and delegates to Advance/Render.
• Sampler examples updated for the stereo separation parameter; sampler render pipeline refactored into isolated, testable steps with hardware synth handling abstracted.
• Channel display output split into MVVM-style layers to decouple UI rendering from player state.
• Generics pared back where unused and reflection-based option handling replaced with typed options; OPL2 hardware mix tolerates mismatched sample lengths.
• Playback/feature option parsing and tracker-version quirks wiring refined for deterministic sequencing, loop handling, and tick/render behaviors.
• Stereo mixing now properly accounts for singular power factor, which matches modern stereo mixing behaviors.
• Stereo separation is now properly handled across all calculations, not just matrix mixing.
• Dolby Pro Logic II surround sound now mixed correctly in Stereo mode.

Removed

• Unused type parameters and reflection-based option paths trimmed from effects/helpers and tracing entries.