Minimize IWYU include surface

[VolkerChristian]

Summary

• add directly required standard and project includes
• remove redundant transitive logging and implementation includes
• preserve intentional public include surfaces with narrow IWYU pragmas
• replace removable header dependencies with forward declarations

Validation

• diff contains only includes, IWYU pragmas, and forward declarations
• git diff --check passes
• full repository CI will validate the branch before merge