Support reification of rotate transformations

[reznakt]

Adds support for reifying rotate() transformations, converting shapes that cannot express a rotation into paths.

Behavior

• reify() now applies rotate(), not just translate()/scale().
• Exact on circle, line, polyline, polygon and path.
• Exact on rect and ellipse at multiples of 90 degrees.
• Pushed down through containers.
• rect/ellipse at any other angle are converted to <path> in place.
• text, image, nested viewports and percentage coordinates keep the transformation, as before.
• Reifying a rotation requires the affected lengths to be convertible to user units and not be percentages, because a rotation mixes the x-axis and the y-axis.

Signatures

• reify() returns Element instead of None — itself, or the replacement <path>.
• Ignoring the return still works, except for a converted root element, which the caller has to rebind.
• Element.replace_with(new) — new method, swaps an element for another in its parent.

Contracts that changed

• BasicShape.to_path() returns a detached element (parent is None) carrying deep copies of its children. It previously returned one attached to a deep-copied clone of the whole document, with the children dropped.
• Rect.to_path_data() emits a closed subpath, omits degenerate A0,0 arcs on square corners, and clamps rx/ry to half the width/height. Command counts and stroke joins differ from before.
• swap_transforms() raises SvgTransformSwapError for a rotation and a non-uniform scale. It used to return a mathematically wrong pair: the identity holds only when the scale is isotropic or the rotation is a multiple of 180 degrees.
• swap_transforms() now also handles rotation ↔ rotation about differing centres.
• Reifiable includes Rotate.

No removals; the only source-level break is code that annotated reify() as returning None.

[codecov]

Codecov Report

❌ Patch coverage is 91.82390% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.60%. Comparing base (9cca15b) to head (43028cb).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
svglab/entities.py	87.83%	4 Missing and 5 partials ⚠️
svglab/elements/elements.py	94.52%	1 Missing and 3 partials ⚠️

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   91.27%   91.60%   +0.33%     
==========================================
  Files          32       32              
  Lines        3495     3623     +128     
  Branches      364      383      +19     
==========================================
+ Hits         3190     3319     +129     
+ Misses        199      195       -4     
- Partials      106      109       +3     

Files with missing lines	Coverage Δ
svglab/attrparse/transform.py	93.00% <100.00%> (ø)
svglab/elements/traits.py	100.00% <100.00%> (ø)
svglab/models.py	94.11% <100.00%> (ø)
svglab/elements/elements.py	97.47% <94.52%> (-0.07%)	⬇️
svglab/entities.py	84.20% <87.83%> (+1.58%)	⬆️

... and 2 files with indirect coverage changes