Implement LongSpan<T> constructor and indexed access

[coderabbitai]

Summary

Implement the missing runtime functionality for LongSpan<T>.

Required changes

Update src/LongSpan.cs so that LongSpan<T> can represent and expose a valid memory range.

• Add a constructor that initializes _reference and Length.
• Add a bounds-checked indexer.
• Return indexed elements by using Unsafe.Add(ref _reference, index).
• Preserve IsEmpty as a check of Length.

Rationale

The current type has no constructor or element accessor. _reference and Length therefore cannot be initialized for useful instances, and callers cannot access span elements.

Affected area

• src/LongSpan.cs

Acceptance criteria

• LongSpan<T> initializes its reference and length through a public constructor.
• The indexer returns the element at a valid index.
• The indexer rejects indexes below zero and indexes greater than or equal to Length.
• IsEmpty remains true only when Length is zero.

Backlinks

• Originating pull request: Add IMemorySlice interface and update IAllocator methods #8
• Originating review comment: Add IMemorySlice interface and update IAllocator methods #8 (comment)
• Requested by: @vix127