Context

The INT8 core and the f32 fixed-point layer are implemented and bit-exact
(see src/lib.rs, matmul_via_slices and matmul_f32). Current tests cover integer
and on-grid f32 inputs.

Task

Extend the test suite to cover the FP64 reconstruction target and larger
dynamic ranges: mixed-magnitude f32 inputs, and a path that documents where
gridding rounds vs. stays exact.

Acceptance criteria

• Tests for varied-magnitude inputs (not just on-grid), asserting the documented
  exactness boundary.
• A short note in the test documenting when reconstruction is exact vs. rounded.

Pointer

Builds directly on the existing tests in src/lib.rs.
Difficulty: easy

See CONTRIBUTING.md for dev setup and PR conventions. No CLA required (MIT).