MathUtils: declare fastATan2's Pi where its lambdas can use it

[ktf]

The local constexpr T Pi sits outside the two lambdas that read it. For a builtin
T every use is an lvalue-to-rvalue conversion on a constant, so Pi is not
odr-used and needs no capture. For a class type the operator call odr-uses it,
which a lambda with no capture-default cannot do.

Capturing it would not help either, because C, Pi025 and Pi075 are constexpr and
a captured variable is not a constant expression. Declaring Pi inside each lambda
satisfies both, and costs nothing: it is folded at compile time either way.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 81285ab at 2026-10-13 09:15:

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