Add clip-path with basic shapes

[jhaygood86]

Feature

Adds clip-path (CSS Masking 1 §5.1) with the <basic-shape> functions (CSS Shapes 1 §3):

clip-path: none;
clip-path: circle(50% at center);
clip-path: ellipse(closest-side farthest-side at 30% 40%);
clip-path: inset(10px 20px round 5px);
clip-path: polygon(evenodd, 0 0, 100% 0, 50% 100%);

Implementation

ClipPathValueConverter accepts none or one of polygon() / inset() / circle() / ellipse(), validating each function's argument grammar and preserving the authored text:

• polygon — optional <fill-rule> (nonzero/evenodd), then a comma list of <length-percentage> <length-percentage> vertices
• inset — 1–4 <length-percentage> offsets, optional round <border-radius>
• circle — optional <shape-radius> (a non-negative length-percentage, closest-side, or farthest-side), optional at <position>
• ellipse — same, with two radii

Negative radii, the wrong number of radii, incomplete vertices, a bare round, and unknown values are all rejected.

The <geometry-box> and url() reference-box forms are out of scope.

Tests

29 cases in PropertyTests/ClipPathProperty.cs: 16 legal shapes, 11 malformed (banana, circle(-5px), ellipse(50%), three-radius ellipse, inset(), inset(10px round), five-offset inset, incomplete polygon vertex, empty polygon, circle(at), two-radius circle), and a text-preservation check.

The legal and illegal cases fail on master (the property is unknown). The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.