0.3.0: correctness fixes, doc improvements, and test coverage

Breaking: remove redundant k and n public fields from SplineCurve
(already encoded as const generics K and N).

Fixes:

• plot: step_by(1) → step_by(2) for correct 2D axis bounds
• error messages reported wrong required coefficient count
• eval() now populates all N dimensions (previously only last dim)
• remove hardcoded spectral axis formatter from generic plot utility

Clean-up:

• remove unused serde/serde_json hard dependencies
• remove dead deboor_derivative stub and duplicate test assertions
• replace if xy.is_some() { xy.unwrap() } with if let Some(xy) = xy

Docs:

• full doc comments on SplineCurve, new/try_new, evaluate, eval, deboor,
  transpose, SplineCurves, and all plot helpers
• document DE_BOOR_SIZE compiler workaround in eval return type
• fix typos ("Calulates", "firt", "splnies")

Tests: 6 → 16 tests, adding coverage for error paths, clamping,
eval out-of-range/cache-reset, 2D curves, transpose, and key-not-found