Phase 3 / M4: Add deterministic smoothing and automation lanes #9

Objective

Implement bounded, sample-accurate smoothing and built-in automation curves shared by real-time and offline rendering.

Scope

• Implement deterministic audio-engine smoothing, including zero-duration steps.
• Add automation lanes/segments/handles with linear, exponential, smoothstep, and hold curves.
• Define exact endpoint and frame-inclusivity semantics.
• Implement both ramp forms, automate, cancel-automation!, inspection, and traces.
• Enforce one authoritative lane per target, deterministic replacement/rejection, and value precedence.
• Enforce bounded lanes/segments/horizon and explicit queue/limit errors.
• Test cancellation, overlap, monotonicity, endpoints, no overshoot, exponential constraints, and block-size invariance.
• Add controlled-gain/filter sweep fixtures and internal test-only lane sampling where audio analysis is ambiguous.

Acceptance criteria

• Required curves produce exact deterministic endpoints at exact frames.
• Smoothing and automation are independent of callback block size and never produce NaN/Inf.
• Spectral sweep and automation audio validation pass; active lanes remain bounded.

Spec: phase-3.md §§12–13, 25–33, 35–36, 40 (Milestone 4), 42.