Replace O(n^2) interval merging with sort-then-merge#15
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mergeIntervals used a nested loop to find merge candidates, making union-heavy operations cubic when combined with iterative Union calls in npm || and hex or parsing. Now sorts intervals by lower bound and merges in a single linear pass. The npm and hex parsers also collect all intervals first instead of merging incrementally.
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`mergeIntervals` used a nested loop to find merge candidates, making it O(n^2). Combined with the iterative `result.Union(r)` pattern in npm `||` and hex `or` parsing, this produced O(n^3) total time for constraint strings with many disjunctions.
Two changes: