[masarray]

Summary

Strengthens the existing OBS/VST3 execution contract without changing its ADR/ticket ordering or product gates.

Added failure architecture

• no exception-driven control flow in filter_audio, Rack DSP workers, or shared process-block hot paths
• expected helper/vendor/block/runtime failures use compact Result/status values with deterministic fail-dry behavior
• Windows/COM/VST3/vendor exceptions are contained at non-real-time helper/control boundaries
• real-time diagnostics use tiny fixed-size events/counters in bounded non-blocking queues/rings
• formatting/logging/aggregation is off the audio path and error storms are deduplicated/rate-limited
• diagnostic failure cannot block DSP, trigger restart loops, or crash OBS
• candidate state promotion is transactional and retains last-known-good state on failure

Documentation-only change; execution order and runtime behavior are unchanged.