Typst speeds up convergence check 334x, fixing 14-min compile regression

A performance regression in Typst 0.15 caused a 300-page document to compile in 14 minutes instead of 1m17s. The bottleneck was a new introspection analysis step that iterated over 11.6 million entries without deduplication. By deduplicating and reducing iterations to just 3 distinct introspections, the fix achieves a 334x speedup.