fix(set): correct IsDisjoint logic and optimize memory allocation

[mohitjoshi-hey]

This PR fixes a logical bug in IsDisjoint and addresses the TODO: maybe optimize? comment.

The Bug:
Previously, IsDisjoint returned true if the intersection size was > 0, which is the mathematical opposite of disjoint (sharing zero elements).

The Optimization:
Instead of fixing it by changing the condition to == 0, I rewrote the method using s.Each(). The previous implementation called s.Intersection(other), which forced a full O(N) memory allocation to clone the set just to check for a single overlap. The new implementation checks for overlaps in-place, returning immediately with O(1) auxiliary space.

Changes:

• Fixed IsDisjoint logic to return true only if zero elements are shared.
• Rewrote the check to use s.Each() for zero-allocation performance.
• Added ExampleSet_IsDisjoint in set_test.go to verify both true and false disjoint conditions while matching the existing testing style.

Testing:

• go test ./set/... passes locally.
• go fmt applied.

Closes #63