[enumeratio]

∫ sin³x dx is cos³x/3 − cos x, ∫ sin³x cos²x dx is cos⁵x/5 − cos³x/3, ∫ sin⁴x dx is 3x/8 − 3/16·sin 2x − ¼·cos x·sin³x; they were Integrate. Fixes #578

Rules (symbolic/antiderivative.ts):

• integrateSinCosPower(fn, index) reads the integrand as c·sinᵐ(u)·cosⁿ(u) (a constant factor, a Negate, a Divide by a constant, one linear argument u = a·x + b with a from linearIndexCoefficient, integer powers 1…SIN_COS_POWER_LIMIT = 40) and returns the primitive divided by a. For m = 2k + 1 it is −Σⱼ C(k, j)(−1)ʲ cosⁿ⁺²ʲ⁺¹u/(n + 2j + 1), for n = 2k + 1 it is Σⱼ C(k, j)(−1)ʲ sinᵐ⁺²ʲ⁺¹u/(m + 2j + 1) (sin² + cos² = 1).
• sinCosEvenPower(u, m, n), both even and m + n ≥ 4 (sin², cos² stay the table's): A&S 4.3.127–4.3.128 as a loop, not a recursion, then ∫du = u.
• It is tried in antiderivativeCandidate after the established antiderivative left an Integrate and before logarithmicPrimitive, so no result that already resolved changes (∫ sin x cos x, ∫ sin²x, ∫ sin x · cos x composites, the table entries).
• A power above 40, a non-integer, complex, symbolic or negative power, an argument that is not linear (sin³(x²)) and different arguments in the two factors (sin³x·cos 2x) are not read, and the call stays as it was.

Definite integrals are unchanged (∫₀^π sin³x dx gave 4/3 already, by another route); the degree angular unit scales by 180/π through withRadianConvention. Not compiled (Integrate is not).

Wolfram agrees (its indefinite forms are multiple-angle spellings of the same functions up to a constant).

Tests: test/compute-engine/trig-power-integrals.test.ts (new): seven exact primitives by string (sin³, cos³, sin³cos², sin²cos³, sin³cos³, 3 sin³(2x + 1), cos³/2); for sixteen (m, n, u) (odd, even and mixed, u = x, 2x + 1, 3x, x − 1) the derivative of the result equals the integrand at three points (central difference, 1e-6); definite values (4/3, 3π/16, π/8) under evaluate() and .N() (sin³ was already 4/3); degree mode; and six shapes that are not read and stay Integrate. No existing test changed; the 160-odd test files that name Integrate or \int together with Sin or Cos pass.

Overlap with the open PRs. None; it adds a function and one call in antiderivativeCandidate.
Open question: the even-even case adds a sin(2x) spelling through evaluate() of sin x cos x (∫sin⁴x prints with sin(2x) and cos x·sin³x); a primitive in the sin(kx) multiple-angle form (as Wolfram's) is also possible, and needs TrigReduce on the result.