[abadams]

Copilot says (description written by GitHub Copilot on behalf of @abadams):

is_monotonic's DerivativeBounds only handled a Mul where one operand is a constant, and a Div by a constant. Anything symbolic, like x / max(k, 1) with a Param k, fell back to "unknown", so sliding window and storage folding gave up on it.

This PR generalizes both cases:

• Mul: if one operand doesn't depend on the var (its derivative bounds are exactly zero), the result is the other operand's derivative bounds times the constant_integer_bounds of the independent operand.
• Div: if the denominator doesn't depend on the var and constant_integer_bounds proves it's strictly positive or strictly negative, the quotient moves the same way as the numerator (or the opposite way for a negative denominator), and by at most as much. Rounding matches the existing constant-denominator case: floor for the min, ceil for the max. Both bounds are clamped to include 0, because a step in the numerator can leave the quotient unchanged.

This is a precision improvement, not a miscompile fix. It lets sliding window slide over footprints like f(x / max(k, 1)). In the new sliding_window test, f is computed 35 times instead of 200. is_monotonic gets new check cases.

Found while fuzzing the sliding-window rewrite stack (#9648); split out because it applies to main independently.

Checklist

• Tests added or updated
• Documentation updated (if public API changed) — N/A
• Benchmarks/performance impact considered — N/A

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.97%. Comparing base (8ac8ab4) to head (65709de).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Monotonic.cpp	91.30%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9664      +/-   ##
==========================================
- Coverage   73.01%   72.97%   -0.04%     
==========================================
  Files         266      266              
  Lines       83355    83374      +19     
  Branches    20474    20480       +6     
==========================================
- Hits        60858    60840      -18     
- Misses      16152    16153       +1     
- Partials     6345     6381      +36     

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