Phase 3 / M2: Implement persistent controlled-Sointu adapter #7

Objective

Allow compiled Sointu unit parameters to receive persistent host control values without rebuilding or updating the patch for each write.

Scope

• Introduce the internal controlled-synth abstraction and generation-specific binding table.
• Implement persistent external-control storage and documented schema-driven effective-value calculation.
• Support instrument-wide and per-voice writes, reads, resets, range transforms, and binding-count inspection.
• Preserve normal Sointu behavior byte-for-byte/tolerance-equivalently when no controls are declared.
• Keep allocations, blocking locks, logging, Lisp evaluation, and compilation out of the render callback.
• If needed, add the minimal isolated Go VM patch, regression tests, and upstream/removal documentation identified in M0.
• Add adapter tests using the real pinned Sointu implementation.

Acceptance criteria

• External values persist until replaced/reset/invalidated and measurably affect rendered audio.
• Instrument and voice binding writes work without Synth.Update or patch recompilation.
• Uncontrolled Phase 1/2 patches render unchanged.

Spec: phase-3.md §§5–6, 9, 18.6, 30–33, 40 (Milestone 2), 41–42.