clean up index helpers and repo-level md files

[kylecarow]

Summary

• Splits src/strategy/traits.rs into trait definitions (unchanged) and a new
  src/strategy/utils.rs holding the grid/index search helpers those traits'
  interpolate impls call into, matching the split the in-progress cubic-spline
  branch (CubicSpline #16) already landed independently.
• Renames find_nearest_index → locate_lower_index (public, no deprecation shim;
  pre-1.0, no stability guarantee yet), step_index → locate_step_index,
  uniform_lower_index → locate_lower_index_uniform.
• Folds the out-of-range clamp into locate_lower_index itself instead of leaving it
  hand-inlined at every Linear call site. A first draft of Multi-channel ("shared-grid") interpolation support & DynInterpolator/DynInterpolatorMulti #19's multi-channel
  Linear override had reinvented this search without the clamp and panicked on
  extrapolated points; this closes that gap for good.
• Adds AxisLocation<T> + locate_axis, and uses them to replace the duplicated
  clamp-array/exact_index logic in Linear's 1D/2D/3D interpolate (the ND impl
  keeps its own dimensionality-reduction logic, just drops its now-redundant manual
  clamp).
• Renames CHANGELOG → CHANGELOG.md and LICENSE → LICENSE.md so GitHub renders
  them instead of showing raw Markdown as plain text.

Test plan

• cargo build --all-features
• cargo test --all-features (68 unit tests, 15 doc tests, all passing)
• cargo clippy --all-features --all-targets (no new warnings vs. main)
• cargo fmt --check