feat: multiplicative quality normalization and propagation across transform chains

[Copilot]

Transform quality was accepted as an unclamped f32, allowing invalid values outside [0.0, 1.0], and TransformPath had no direct unit tests for its multiplicative quality computation.

Changes

Quality normalization — TransformEdge::new()

• Clamps the quality argument to [0.0, 1.0] at construction time, enforcing the invariant regardless of caller input

// Before: quality stored as-is (could be negative or > 1.0)
Self { from, to, cost, quality }

// After: silently clamped to valid range
Self { from, to, cost, quality: quality.clamp(0.0, 1.0) }

Quality propagation tests — pathfinding.rs

Added a #[cfg(test)] module directly on TransformPath::from_steps() covering:

• Single-step quality and cost identity
• Two-step multiplicative product (0.9 × 0.8 = 0.72)
• Three-step chain propagation (0.9 × 0.8 × 0.7 = 0.504)
• Perfect-quality path (1.0 × 1.0 = 1.0)
• Zero-quality edge collapsing entire path quality to 0.0

Normalization boundary tests — transform_edge.rs

Explicit tests for values above 1.0, below 0.0, and at exact boundaries.