Use `FLOAT_EQ_EPSILON` for equality/inequality operators and
division-by-zero guards in formula evaluation to ensure consistent
semantics.
Replace `Vec` with `IndexSet` for stem collection in `recompute_formulas`
to improve performance from O(n²) to O(n).
Add regression tests for epsilon-based comparison and stem collection
performance.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)
Edward Langley
45bfe2c4c7