Remove stale dual_fmt.h references from documentation

Michael Teschrequested to merge
docs/remove-dual-fmt-references into master

Summary

  • Remove references to duals/dual_fmt.h from header tables in duals/dual, README.md, and CLAUDE.md
  • Remove the dual_fmt.h update step from TODO-multidual.md checklist
  • fmt support is implemented inline in duals/dual via CPPDUALS_LIBFMT; dual_fmt.h was never committed

Test plan

  • Docs rebuild clean — no dual_fmt.h references in generated Doxygen output
  • Verify CI passes

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