Integrate xad-codegen JIT backend

[auto-differentiation-dev]

Summary

• Update QuantLibAAD to use xad-codegen for JIT compilation
• Available under a separate commercial license
  (XAD Enterprise)

Changes

• New CMake option QLAAD_ENABLE_CODEGEN (default OFF)
• Updated codegen discovery: checks for xad_codegen target from XAD
• Updated all source files to use xad::codegen::CodegenBackend API
• Renamed benchmark output keys to CODEGEN_ / CODEGENAVX_
• Removed codegen-dependent CI jobs (require commercial license)
• Updated README with codegen description and enterprise support link

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