Move generic inference/specialization family into MLIRGenGenerics.cpp (review doc 1)

[ASDAlexander77]

Fourth mechanical slice of the §1 TU split from docs/MLIRGen-refactoring-review.md (follows #217, #218, #219).

What

• MLIRGenGenerics.cpp (new): the contiguous generic-type machinery — the tryInfer* family, inferType/inferTypeFuncType, instantiateSpecializedFunction (both overloads), instantiateSpecializedClassType/InterfaceType, resolveGenericParam(s)FromFunctionCall, appendInferredTypes, mlirGenSpecialized, and the two specialization mlirGen wrappers (26 methods, ~1,400 lines) — defined out-of-line.
• MLIRGenImpl.h shrinks from 21,225 to 19,909 lines.
• The template member tryInferTupleFields<T> stays inline in the header.

Same mechanical recipe as the previous slices: bodies byte-identical, MLIRGenImpl:: qualification added, default arguments stay on declarations only.

Verification

• Debug build clean.
• MLIRGenTests 12/12.
• Full ctest suite: 681/681 (339 JIT + 342 AOT).

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