COMP: Forward-declare itk::Indent in itkPrintHelper.h

itkPrintHelper.h is included from itkMacro.h. Common ITK headers include
itkIndent.h, which itself includes itkMacro.h, which transitively includes
itkPrintHelper.h. When itkIndent.h is the include-chain entry point, the
recursive inclusion of itkIndent.h from itkPrintHelper.h's prologue is a
header-guard no-op — class itk::Indent has NOT yet been declared at the
point itkPrintHelper.h is parsed.

Most toolchains tolerate this because the only reference to Indent in
this file is a function parameter declaration (`const Indent & indent`),
and many parsers accept forward use of an undeclared type at parse time.
Stricter configurations (Apple Clang dbg, AppleClang ASanUBSan, ClangMain,
TSan, gcc 11.4 TBB-Debug, MSVC 2022 Static-DebugTBB, Valgrind, Coverage)
reject it as:

    itkPrintHelper.h:97:44: error: unknown type name 'Indent'

The fix is the same pattern already used a few lines above for NumericTraits:
add an explicit forward declaration of class Indent inside namespace itk
before the namespace itk::print_helper block. Every PrintNumericTrait()
call site already #includes itkIndent.h directly or transitively (they
need to stream `os << indent`), so the forward declaration is sufficient
for the template's parameter list to parse; the full Indent definition is
visible at every instantiation site.

Symptom across 8+ Nightly / Expected Nightly platforms (2026-05-11):
CE=1 / TF=0 / TN=~2660 — one compile error in ITKCommon blocks the
entire test grid.

Author: hjmjohnson

Modules/Core/Common/include/itkPrintHelper.h

@@ -38,6 +38,13 @@ namespace itk
 // sites already #include "itkNumericTraits.h" directly or transitively.
 template <typename T>
 class NumericTraits;
+// Forward declaration for the same reason: itkIndent.h pulls in itkMacro.h
+// which pulls in itkPrintHelper.h, so when itkIndent.h is the include-chain
+// entry point the recursive include of itkIndent.h above is a no-op and
+// class Indent is not yet visible.  Every PrintNumericTrait() instantiation
+// streams `os << indent` and therefore needs the full Indent definition,
+// but call sites already #include "itkIndent.h" directly or transitively.
+class Indent;
 } // namespace itk
 
 namespace itk::print_helper