Add LongSpan<T> ref struct for memory regions

[vix127]

Introduced LongSpan to represent contiguous memory, supporting both managed and unmanaged sources. Added field for length, constructors, Length and IsEmpty properties, indexer with bounds checking, and XML documentation.

Summary by CodeRabbit

• New Features
  • Added support for representing contiguous managed, native, and stack memory regions.
  • Added constructors for creating spans from references or unmanaged memory pointers.
  • Added bounds-checked indexed access using 64-bit indices and by-reference element access.
  • Supports memory regions larger than standard 32-bit collection limits.
  • Added validation for invalid lengths and unsupported reference-containing types.

[coderabbitai]

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ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between 47a3315 and 6b723e6.

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• src/LongSpan.cs

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📝 Walkthrough

Walkthrough

LongSpan<T> now represents managed, native, or stack memory with a byref and a long length. It adds reference and pointer constructors, validates pointer inputs, and provides bounds-checked indexed access.

Changes

LongSpan API

Layer / File(s)	Summary
LongSpan representation and properties src/LongSpan.cs	LongSpan<T> stores its length as a long and exposes Length and IsEmpty.
Memory construction and indexed access src/LongSpan.cs	The type adds reference and unmanaged pointer constructors. The pointer constructor rejects invalid lengths and managed-reference types. The indexer returns a reference and throws for invalid indices.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related issues

• vix127/SharpAllocators issue 9 — Describes the same LongSpan<T> constructor and bounds-checked indexed access implemented by this pull request.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the introduction of the LongSpan ref struct for memory regions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

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• Create PR with unit tests
• Commit unit tests in branch Implement-longspant-constructor-and-indexed-access

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@src/LongSpan.cs`:
- Around line 83-88: Update the bounds check in LongSpan’s indexer to compare
the int index against the full long _length without casting _length to uint,
while preserving rejection of negative and out-of-range indices and the existing
Unsafe.Add access.
- Around line 54-67: Update the LongSpan(void* pointer, long length) constructor
to reject a null pointer when length is greater than zero before assigning
_reference. Preserve valid zero-length spans, and keep the existing length and
reference-type validation behavior unchanged.

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ℹ️ Review info

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Configuration used: defaults

Review profile: CHILL

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Run ID: 9f9fe6f5-d91a-4ea9-978d-6b6e8a04f014

📥 Commits

Reviewing files that changed from the base of the PR and between a7a77c8 and 47a3315.

📒 Files selected for processing (1)

• src/LongSpan.cs

[vix127]

Implement changes before merging