fix: make ContiguousSet and InsertionOrderSet IntersectWith/SymmetricExceptWith linear [patch]

[matt-edmondson]

Fixes #83

What was wrong

ContiguousSet<T>.IntersectWith and SymmetricExceptWith called the public Remove(item) for each element they dropped. Each call rescans items from index 0 and shifts the tail, so dropping most of a set costs O(n²). InsertionOrderSet<T> had the same shape, with one List.RemoveAt per dropped element.

Change

• ContiguousSet: a private RemoveWhere makes one compaction pass. It keeps a write index, removes dropped items from uniquenessSet, clears the vacated tail when SlotClearing.IsNeeded<T>() says to, and bumps version once, only if something was removed. Both operations use it.
• InsertionOrderSet: List.RemoveAll with the same predicates.
• Both are now O(n + m) and keep the surviving elements in insertion order. Items found only in other are still appended afterwards, as before.

Tests

• *_RunsInLinearTime: n = 100k for ContiguousSet and n = 200k for InsertionOrderSet, each with a 1.5 s budget.
  • Against the old code in a Debug build they fail at 33 s and 32 s (ContiguousSet) and 15 s and 14 s (InsertionOrderSet).
  • With the fix, all four run in about a second combined, including test host startup.
• New order and consistency tests:
  • Survivors keep insertion order, and items that were only in other are appended after them.
  • Contains and Add agree with the new contents.
  • An enumerator open during the operation is invalidated.
• Full suite: 447/447 pass on net10.0. The library builds clean on every target.

This is a separate change from #109 (#78), which touches AsSpan in the same file. The two changes do not overlap.

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https://claude.ai/code/session_01J6q8NVYVFUeBVAH6tJCHnU

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.2% Coverage on New Code
1.9% Duplication on New Code

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