Make unsafe code opt-in by the consumer

[Tyrrrz]

No description provided.

[Copilot]

Pull request overview

This pull request makes unsafe code opt-in for consumers of the PolyShim package, rather than automatically enabling it. Previously, the package automatically enabled AllowUnsafeBlocks for all consumers, which could have security and compatibility implications.

Key changes:

• Unsafe code is now opt-in: consumers must explicitly set <AllowUnsafeBlocks>true</AllowUnsafeBlocks> to use polyfills that require unsafe code (e.g., Span with stackalloc)
• Conditional compilation is used to only include unsafe polyfill features when the consumer has enabled unsafe blocks
• Comprehensive documentation added explaining when and how to enable unsafe code

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Readme.md	Added documentation section explaining unsafe code requirements, with examples showing how to enable it and when it's needed. Updated compatibility package example to better demonstrate System.Memory usage.
PolyShim/PolyShim.props	Removed automatic AllowUnsafeBlocks setting that was previously forced on all consumers
PolyShim/PolyShim.targets	Added conditional compilation constant ALLOW_UNSAFE_BLOCKS when consumer enables unsafe blocks
PolyShim/PolyShim.csproj	Added AllowUnsafeBlocks for building the PolyShim package itself
PolyShim/NetCore21/Span.cs	Changed unsafe constructor to be conditionally compiled based on ALLOW_UNSAFE_BLOCKS define. Simplified pointer arithmetic implementation from Unsafe.Read to direct pointer indexing.
PolyShim.Tests/PolyShim.Tests.csproj	Enabled AllowUnsafeBlocks for test project to test unsafe polyfill features

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (3766cff) to head (424b744).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@     Coverage Diff      @@
##   master   #69   +/-   ##
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