Fix Tessellation error for holes that share the same leftmost vertex with an outer ring vertex

[roblburris]

Description

Apologies in advance if this is garbage, but I had Claude make an attempt at fixing the Tessellation error for the geometry in #15695:

When holes are eliminated left-to-right, splitPolygon creates copies of bridge endpoints in the ring. If multiple holes share the same leftmost vertex, the ring accumulates multiple copies of that vertex (all with the same idx). The angle-based getSharedInsideVertex tiebreaker could select the wrong copy when merging subsequent holes, producing a malformed ring that fails ear-clipping.

This is fixed by taking the first match in ring order from outerNode for leftmost shared vertices instead of using the angle-based tiebreaker. When multiple copies exist, outerNode is updated to the bridge position so subsequent holes start scanning from the correct place.

All tests pass including a new one for the geometry linked in #15695. The description of the problem and fix seem correct but I'll defer to people more familiar with the nuances of Tessellator to verify this 😄