Add struct enumerator and IEnumerable<int> to IntSet

[marius-bughiu]

Summary

Mirrors the dictionary-side enumeration work (PRs #47 / #48) on the set side. IntSet<THasher> now ships a struct Enumerator, a public GetEnumerator(), and IEnumerable<int> conformance, so foreach (int item in set) works without a wrapper and IntSet is ready for LINQ.

The out-of-band zero entry is yielded first when present, matching how the dictionaries surface their out-of-band default-key entry. A _version counter is bumped on every entry-point structural mutation (Add, TryAdd, Remove, Clear); the struct enumerator captures it on construction and re-checks it from MoveNext / Reset, so concurrent modification fast-fails with InvalidOperationException exactly the way BCL HashSet<T> does. No-op mutations (re-adding an existing item, removing an absent item, Clear on an empty set) deliberately do not bump the version, so active enumerators stay valid across them — also a regression target.

This is the first slice of issue #23 — CeleritySet<T, THasher> follows in a separate PR (the open-generic T path needs IsDefaultValue care that's worth its own review). Together they unblock the future IEnumerable<T> constructor on the sets and the post-1.1.0 IReadOnlySet<T> interface.

Changes

• src/Celerity/Collections/IntSet.cs — add IEnumerable<int> interface, _version field, version bumps on TryAdd / Remove / Clear, public GetEnumerator(), struct Enumerator with BeforeZero / InArray / Done state machine.
• src/Celerity.Tests/Collections/IntSetEnumerationTests.cs — 25 new tests (see test plan).
• CHANGELOG.md, ROADMAP.md, ISSUES.md — documented under issue Add SIMD optimizations #23.

Test plan

• dotnet build — 0 errors, no new warnings beyond the existing pre-existing analyzer noise.
• dotnet test — 431 / 431 pass (25 new tests in IntSetEnumerationTests, all 406 prior tests still green).
• Empty / single / many-entry enumeration.
• Zero-first-when-present ordering contract.
• Removal, zero-removal, and Clear reflected in subsequent enumeration.
• Multi-resize survival from a tiny initial capacity (200 entries from capacity: 4).
• Mutation-during-enumeration throws on MoveNext and Reset (insert, zero-insert, remove, zero-remove, clear).
• No-op mutations (duplicate TryAdd, absent Remove, empty Clear) leave active enumerators valid.
• Reset reusability and post-exhaustion Current reset.
• Generic IEnumerable<int> and non-generic IEnumerable parity.
• Open-generic path covered via IntSet<Int32Murmur3Hasher>.
• LINQ smoke test (Count / Sum / Contains) confirming the boxed enumerator path.