Phase 3 / M0: Verify baseline and assess controlled-Sointu architecture

Objective

Establish a clean Phase 1/2 baseline and decide how persistent host controls will be implemented before Phase 3 development begins.

Run and record the existing make acceptance result before Phase 3 changes.
Review scheduler, audio-thread, patch-generation, and transactional-install boundaries.
Inspect the pinned Sointu version for persistent external-control extension points.
Document whether an upstream-compatible adapter is sufficient or a minimal Go VM patch is required.
If a patch is required, define its isolation, regression-test, third_party/patches documentation, and upstream/removal plan.
Capture control-write, render, and patch-update benchmark baselines.

Phase 1 and Phase 2 acceptance tests pass unchanged.
A feasibility report documents the effective-value model and confirms ordinary control writes will not use Synth.Update or patch recompilation.
Audio-thread safety constraints and baseline performance numbers are recorded.

Spec: phase-3.md §§2, 5–7, 33, 40 (Milestone 0), 41.