Implement PartialEq for Position and add frame-tagged matrix primitives

CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [unreleased]
+
+### Added
+- New frame-tagged 3×3 matrix primitives in `affn::matrix3`: `FrameMatrix3<F, T>` for general matrices and `SymmetricFrameMatrix3<F, T>` for symmetry-preserving covariance-style blocks.
+- Rotation similarity helpers on the new matrix types: `rotated_by::<G>(&Rotation3)` for frame-changing `R · M · Rᵀ` transforms, with numerical re-symmetrization for `SymmetricFrameMatrix3`.
+- `Rotation3::apply_vec<F1, F2, U>` for rotating typed `Vector`/`Displacement` values while preserving units and retagging the output frame.
+
+### Changed
+- `Position<C, F, U>` now implements `PartialEq`, comparing both Cartesian coordinates and `center_params` so affine points only compare equal when they represent the same location relative to the same parameterized center state.
+
 ## [0.7.0 - 2026-05-09]
 
 ### Removed

src/cartesian/position.rs

@@ -563,6 +563,22 @@ forward_ref_binop! {
     )
 }
 
+// =============================================================================
+// PartialEq
+// =============================================================================
+
+impl<C, F, U> PartialEq for Position<C, F, U>
+where
+    C: ReferenceCenter,
+    C::Params: PartialEq,
+    F: ReferenceFrame,
+    U: LengthUnit,
+{
+    fn eq(&self, other: &Self) -> bool {
+        self.xyz == other.xyz && self.center_params == other.center_params
+    }
+}
+
 // =============================================================================
 // Display
 // =============================================================================

src/lib.rs

@@ -123,6 +123,9 @@ pub mod planar;
 #[cfg(feature = "serde")]
 pub(crate) mod serde_utils;
 
+// Frame-tagged 3×3 matrix primitives
+pub mod matrix3;
+
 // Re-export derive macros from affn-derive
 // Named with Derive prefix to avoid conflicts with trait names
 pub use affn_derive::{