Scalar trait and backends: F32Scalar + DFix64 (Q32.32) with unit tests

[flyingrobots]

Define Scalar trait (core ops, transcendentals via trait fns) and implement:

• F32Scalar (deterministic wrappers, canonicalize -0.0, flush subnormals)
• DFix64 (Q32.32: i64 with i128 accumulators; nearest‑even rounding; saturating overflow in release)

Deliver:

• Unit tests for basic ops and conversions
• Feature flags to compile either backend