[ondrejmirtes]

intdiv() always returned int, because the only extension for it described what it throws. This adds IntdivFunctionReturnTypeExtension, which derives the result range from the integer ranges of both arguments.

• The divisor is split into its negative and positive parts; zero is dropped because dividing by it throws.
• For each numerator × divisor part, real intdiv() is evaluated at the four corners. Truncated division is monotonic in both operands while the divisor keeps one sign, so the corners give the exact bounds. Unbounded ends are handled as ±INF.
• Union members are kept separate, e.g. intdiv(3|int<10, 12>, 2) is 1|int<5, 6>.
• / logic in InitializerExprTypeResolver is not reused: it rounds bounds inwards (int<1, 5> / 2 → int<1, 2>|float), while intdiv() truncates towards zero (int<0, 2>).

Examples: intdiv(int<0, max>, int<1, max>) → int<0, max>, intdiv(int<-7, -1>, 2) → int<-3, 0>, intdiv(7, 2) → 3.

Verified against brute-force intdiv() over 5,041 range pairs: every result is contained, and for bounded ranges the bounds are exact.

Closes phpstan/phpstan#15282
Closes phpstan/phpstan#15347