DOC: Add Modules/Filtering/FixedPointInverseDisplacementField/README.md

Author: hjmjohnson

Modules/Filtering/FixedPointInverseDisplacementField/README.md

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+# FixedPointInverseDisplacementField
+
+In-tree ITK module that computes the inverse of a displacement field using the
+fixed-point iteration of Chen et al. (*A simple fixed-point approach to invert
+a deformation field*, Medical Physics 35(1):81, 2008). Given a field mapping
+space A → B, it produces the field mapping B → A.
+
+Flagship class: `itk::FixedPointInverseDisplacementFieldImageFilter`.
+
+## Origin
+
+Ingested from the standalone remote module
+[**InsightSoftwareConsortium/ITKFixedPointInverseDisplacementField**](https://github.com/InsightSoftwareConsortium/ITKFixedPointInverseDisplacementField)
+on 2026-06-03 via the v4 ingestion pipeline. The upstream repository will be
+archived read-only after this PR merges; it remains reachable at the URL above
+for historical reference.