Optimize duplicate checking

[acgetchell]

src/core/triangulation.rs
1013-1045: Acknowledge O(n²·D) duplicate checking—consider optimization strategy.

The duplicate coordinate check is O(n) per insertion, leading to O(n²·D) construction cost.

Evaluate a keyed/quantized coordinate index for optimization.

Current impact: For moderate triangulations (~1,000 vertices), the overhead is manageable. For larger triangulations (>10,000 vertices), this could become a bottleneck.

Recommendation: Track this as a known performance limitation. If users report slow construction times, implement a spatial hash or grid-based duplicate check as suggested in the comment.

[acgetchell]

Blocked on public topology traversal API (planned for v0.7.0). Will re-evaluate after merge.