Deterministic measurability of the parametrix tuple assembler

The tuple assembler is part of the deterministic kernel layer. Keeping its measurability theorem here prevents continuum and deterministic-parametrix files from importing the random-parametrix measurability module.

theorem Anderson4D.measurable_assemble_prod (m : ℕ) : Measurable fun (p : T4 × T4 × (Fin m → T4)) => assemble p.1 p.2.1 p.2.2

Joint measurability of the external/internal tuple assembler.