Add Rotation::from_euler + as_matrix, resolve issue 001

- scipy/transform.h: add as_matrix(T* m9) public method (quaternion→matrix)
  Uses scipy's exact formulas: R[i,j] = f(x²,y²,z²,w²,xy,zw,...)
- scipy/transform.h: add from_euler(seq, angle) single-axis (std::sin/cos)
- scipy/transform.h: add from_euler(seq, angles[]) multi-axis with correct
  extrinsic (lowercase, reversed compose) / intrinsic (uppercase) handling
  matching scipy's _rotation.pyx Hamilton-product composition order
- pycpp/transform_py.h: add from_euler() Python wrapper that delegates to
  scipy.Rotation.from_euler (numpy/SVML sin/cos → exact quaternion)
- pycpp/transform_py.h: add as_matrix() Python wrapper calling C++ arithmetic
  with exact quaternion → 0-ULP full pipeline
- tests/test_all.py: TestFromEulerAsMatrix (38 tests, 960 assertions, all 0 ULP)
  Key regression: from_euler(z, 5.837569e-06).as_matrix() → 0 ULP
  Covers: near-zero yaw, all 6 extrinsic seqs ×100, intrinsic XYZ/ZYX ×100,
  full roundtrip from_euler→as_matrix→from_matrix→as_euler
- issue/001: marked RESOLVED

Author: peng.li24

README.md

@@ -48,9 +48,19 @@ scipy::ndimage::gaussian_filter1d(src, out.data(), n, sigma);
 // Signal — median filter
 scipy::signal::medfilt(src, dst.data(), n, kernel_size);
 
-// Transform — Rotation (from_matrix + as_euler)
+// Transform — Rotation (from_matrix, from_euler, as_euler, as_matrix)
 auto rot = scipy::spatial::transform::Rotation<double>::from_matrix(R);
 auto euler = rot.as_euler_vec("xyz");  // returns [rx, ry, rz]
+
+// from_euler + as_matrix — single axis (key use case: ego yaw)
+double m9[9];
+auto rot_z = scipy::spatial::transform::Rotation<double>::from_euler("z", yaw);
+rot_z.as_matrix(m9);   // 3×3 row-major rotation matrix
+
+// from_euler — multi-axis extrinsic / intrinsic
+double angles[3] = {rx, ry, rz};
+auto rot_xyz = scipy::spatial::transform::Rotation<double>::from_euler("xyz", angles);
+rot_xyz.as_matrix(m9);
 ```
 
 ### Dependencies
@@ -84,7 +94,7 @@ including extreme values (±inf, NaN, ±0.0, subnormals, saturation inputs).
 | `spatial` | pure C++ / scipy ckdtree | cdist, KDTree | ✅ **0 ULP** |
 | `ndimage` | Python numpy kernel | gaussian_filter1d | ✅ **0 ULP** |
 | `signal` | sort-based median | medfilt | ✅ **0 ULP** |
-| `transform` | delegates to scipy | Rotation | ✅ **0 ULP** |
+| `transform` | delegates to scipy | Rotation.from_matrix, from_euler, as_euler, as_matrix | ✅ **0 ULP** |
 
 Full per-test ULP report: [`doc/ulp_report.csv`](doc/ulp_report.csv)
 (Auto-generated by `pytest tests/test_all.py`, see [doc/ulp_report.md](doc/ulp_report.md) for summary)