Add u-substitution rule (f'(x) * g(f(x)))

[Pooria-Lari]

Goal

Implement a basic u-substitution integrator rule to handle patterns like:

• ∫ 2xcos(x^2) dx = sin(x^2)
• ∫ 3exp(3x) dx = exp(3*x)
• ∫ (2*x)/(x^2+1) dx = ln(x^2+1)

Scope (v0.4)

• Only handle multiplication forms: f'(x) * g(f(x))
• Support g(u) as:
  • sin(u), cos(u), exp(u)
  • 1/u (to produce ln(u))
• No full general Risch algorithm, no integration by parts yet.

Approach

• Use existing diff() + simplify() as a checker:
  Try candidates u inside factors and verify if one factor equals diff(u).
• Extend integrate.py with a new rule before raising IntegrateError.

Acceptance Criteria

• New tests in tests/test_integrate_subst.py (or extend test_integrate.py) pass:
  • roundtrip: simplify(diff(integrate(f))) == simplify(f)
  • for the listed examples above
• Existing tests remain green.

Notes

• Keep implementation conservative: better to reject unknown forms than return wrong answers.