Add IEnumerable<T> constructor on IntSet and CeleritySet

[marius-bughiu]

Summary

Closes #69. Adds an IEnumerable<T> constructor to IntSet, IntSet<THasher>, and CeleritySet<T, THasher> so the sets can be bulk-loaded from an existing sequence — closing the last set-side API-parity gap with the dictionaries (which got the equivalent IEnumerable<KeyValuePair<,>> ctor in 1.1.2).

Unlike the dictionary version, duplicate elements (including duplicate default(T) / zero entries) are silently deduplicated — sets have no duplicate-key contract, so this matches BCL HashSet<T>(IEnumerable<T>).

When the source implements ICollection<T>, its Count is used to size the backing storage; otherwise the caller-supplied capacity parameter is used. The out-of-band default(T) / zero slot is populated correctly via TryAdd on the iteration path.

Test plan

• dotnet build clean (0 errors)
• CI runs SetIEnumerableConstructorTests on Linux / Windows / macOS — local runtime is .NET 10 only and the projects target net8.0, so the test execution gate is CI
• Coverage in SetIEnumerableConstructorTests (mirrors IEnumerableConstructorTests for the dictionaries):
  • Null source throws ArgumentNullException
  • Invalid loadFactor (0f, 1f) still throws ArgumentOutOfRangeException
  • Empty / array / List<T> / non-collection (Enumerable.Range) sources copy correctly
  • Duplicate elements silently dedupe — including duplicate zero / null / default(T) entries
  • Out-of-band zero / null capture from source
  • 500-entry large-source round-trip
  • Source mutated after construction does not affect the set
  • Caller-specified capacity larger than the source count is honored
  • Cross-set copy: build a CeleritySet<int, ...> from an IntSet enumeration
  • Open-generic IntSet<Int32WangNaiveHasher> smoke test