Add StepLower and StepUpper marker strategies with enum wiring and legacy alias compatibility

[kylecarow]

Resolves #23

Summary

This PR adds zero-sized StepLower and StepUpper marker strategies for fixed-direction step interpolation across 1D/2D/3D/ND.

When direction is known at compile time, these markers avoid:

• Step direction-vector allocation
• per-call direction matching in interpolate()

Step remains available and unchanged for:

• mixed per-dimension direction behavior
• direction chosen at runtime

Motivation

Step::from(StepDirection::Lower|Upper) is convenient, but still carries runtime machinery intended for flexible direction handling.
For hot paths with fixed direction, StepLower/StepUpper provide a leaner path.

Changes

• Added marker structs:
  • strategy::StepLower
  • strategy::StepUpper
• Implemented strategy traits directly for both markers:
  • Strategy1D
  • Strategy2D
  • Strategy3D
  • StrategyND
• Kept impl From<StepDirection> for Step intact.
• Added marker variants to all strategy enums:
  • Strategy1DEnum
  • Strategy2DEnum
  • Strategy3DEnum
  • StrategyNDEnum
• Added all manual enum wiring (per dimension):
  • variant declarations
  • From impls
  • init / interpolate / allow_extrapolate dispatch arms
• Updated docs/readme guidance:
  • Step is called out first for mixed per-dimension directionality
  • runtime-selected direction is secondary
  • fixed-direction hot loops are steered to StepLower/StepUpper
• Added backward-compat serde aliases:
  • "LeftNearest" -> StepLower
  • "RightNearest" -> StepUpper

Serde Behavior

• StepLower/StepUpper use unit-struct style serialization (consistent with other marker strategies).
• Legacy aliases are accepted only when deserializing StepLower/StepUpper.
• Alias scope is intentionally not applied to StepDirection, so Step payload format is not broadened.

Compatibility

• No breaking API changes.
• Step and From<StepDirection> for Step are preserved.
• Existing Step wire format remains unchanged.
• Legacy serialized names are supported through marker deserialization aliases.

Testing

Added/updated tests for:

• StepLower/StepUpper behavior in 1D/2D/3D/ND
• enum serde round-trips with new variants
• legacy alias deserialization
• guard that {"Step":[...]} does not accept alias names

Validation run:

• cargo fmt
• cargo test --features serde

All tests and doc-tests pass.