Phase 3 / M1: Add named synth parameter model and symbolic binding plans #6

Objective

Extend defsynth with typed, stable named parameters and compile (param ...) references into deterministic symbolic control-binding plans.

Scope

• Add ParameterID, SynthParameter, scope/rate/curve metadata, defaults, transforms, and canonical serialization.
• Parse and validate :params declarations and (param :id [transform]) references.
• Support one parameter targeting multiple unit parameters.
• Validate finite/ranged defaults, scope, smoothing, curve, target compatibility, and conflicting bindings.
• Emit warnings for unused declarations and helpful errors for undeclared references.
• Add generation-specific immutable binding descriptors while keeping user references symbolic.
• Extend fingerprints and synth-info/controls introspection.
• Add parser, validation, determinism, fingerprint, and error-code tests/fuzz targets.

Acceptance criteria

• Controlled synths compile into stable symbolic binding plans independent of unit order where required.
• Invalid declarations/references produce structured diagnostics.
• Parameter IDs remain stable across compatible recompilation.

Spec: phase-3.md §§6, 8, 24–25, 29–30, 35, 40 (Milestone 1), 42.