Add ContainsValue on IntDictionary, LongDictionary, CelerityDictionary

[marius-bughiu]

Summary

• Adds bool ContainsValue(TValue? value) to all three dictionary types — BCL-parity O(n) linear scan that returns true if any entry's value equals value under EqualityComparer<TValue>.Default, matching Dictionary<TKey, TValue>.ContainsValue.
• Walks the probe table skipping EMPTY_KEY / default(TKey) slots (so the empty-slot default(TValue) payload is not mistaken for a real entry) and, when present, the out-of-band zero-key / default-key slot.
• No allocation on the hot path beyond the cached EqualityComparer<TValue>.Default access. Mirrors the implementation pattern already used by the struct enumerator's slot scan.

This was the smallest remaining gap in the basic BCL surface: callers reaching for ContainsValue had to fall back to the boxed IEnumerable<KeyValuePair<,>> enumeration path, defeating the zero-allocation guarantees the dictionaries otherwise provide.

Closes #73.

Test plan

ContainsValueTests covers all three dictionaries:

• Empty map returns false (including for the default(TValue) lookup — guards that EMPTY_KEY slots are skipped).
• Match found in a regular probe-table slot.
• Match found only in the out-of-band zero-key / default-key / null-string-key slot.
• Missing value returns false.
• default(TValue) (0) lookup on a populated dictionary that contains 0 as a value, separate from the empty-dictionary regression check.
• null-value lookup on a reference-type TValue.
• Duplicate values short-circuit on the first match.
• Survives a 100-entry insert from a tiny initial capacity (covers post-resize slot scan).
• Post-Remove / post-Remove(0) / post-Clear correctly stop reporting the removed values.

Local build: dotnet build clean (no errors). Tests cannot run locally — the SDK on this machine is .NET 10 only and the test project targets net8.0 — but CI runs the matrix on ubuntu-latest, windows-latest, and macos-latest and will exercise them.

🤖 Generated with Claude Code

[marius-bughiu]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Done in ec73ccc. The only conflict was in CHANGELOG.md (the [1.2.0] Added section had both the ContainsValue entries from this branch and the IEnumerable<T> set constructor entries from main). Both sets of entries are now preserved in the merged changelog.