Implement LongDictionary

[marius-bughiu]

Priority: Medium
Phase: 2 - New Collections (Q2 2026)

Description

Create a specialized dictionary for Int64/long keys, similar to IntDictionary but optimized for 64-bit keys.

Requirements

• Optimized memory layout for 64-bit keys
• Custom hasher support
• API consistent with IntDictionary
• Comprehensive unit tests
• Benchmarks comparing to Dictionary<long, T>
• XML documentation

Related

• Similar to IntDictionary implementation
• See ROADMAP.md for context

[marius-bughiu]

Closed by PR #54 (initial LongDictionary implementation), with follow-ups #74 (ContainsValue), #78 (Remove(out value) overload), and #79 (single-probe TryAdd). Implementation lives at src/Celerity/Collections/LongDictionary.cs and is covered by LongDictionaryTests / LongDictionaryCollisionTests.