Add Directory.CreateTempSubdirectory polyfill

[Copilot]

Directory.CreateTempSubdirectory(string?), introduced in .NET 7.0, had no polyfill for earlier targets (.NET Standard, .NET Framework, older .NET Core/5/6).

Polyfill implementation

• Added Directory.CreateTempSubdirectory(string?) to PolyShim/Net70/Directory.cs, alongside the existing CreateDirectory(string, UnixFileMode) member.
• Validates the optional prefix has no directory separator characters, throwing ArgumentException otherwise.
• Creates a uniquely-named empty directory under Path.GetTempPath(), optionally prefixed, retrying on name collisions.

Tests

• Added coverage in PolyShim.Tests/Net70/DirectoryTests.cs for default creation, prefixed creation, and the invalid-prefix exception path.

Docs

• Signatures.md regenerated (via build) to list the new member.

var dir = Directory.CreateTempSubdirectory("myapp-");
// dir.FullName => /tmp/myapp-<random>

[Tyrrrz]

@copilot

[Tyrrrz]

@copilot merge in latest prime and resolve conflicts by regenerating Signatures.md

[Copilot]

Copilot review overview

🟡 Changes recommended

Add separator validation and coverage for the invalid-prefix exception path.

Review effort: Lite
Findings: 1 · 1

Open (2)

• Missing prefix validation enables path traversal · New
• Missing test for prefix separator rejection · New

What changed in this PR

Adds a .NET 7 Directory.CreateTempSubdirectory polyfill for older targets, with tests and generated documentation.

Changes:

• Implements temporary subdirectory creation with optional prefixes.
• Adds creation and prefix tests.
• Updates Signatures.md.
• Outstanding findings: missing separator validation (critical) and missing invalid-prefix test coverage (nit).

File	Summary
Signatures.md	Documents the new API member.
PolyShim/​Net70/​Directory.cs	Implements the temporary-directory polyfill.
PolyShim.Tests/​Net70/​DirectoryTests.cs	Tests directory creation and prefixes.

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[Copilot]

@copilot merge in latest prime and resolve conflicts by regenerating Signatures.md

Merged prime and regenerated Signatures.md in merge commit 5cc8d62.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.76%. Comparing base (bec589e) to head (5cc8d62).

Files with missing lines	Patch %	Lines
PolyShim/Net70/Directory.cs	42.85%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #146      +/-   ##
==========================================
- Coverage   87.77%   87.76%   -0.02%     
==========================================
  Files         342      342              
  Lines        7443     7469      +26     
  Branches      539      542       +3     
==========================================
+ Hits         6533     6555      +22     
- Misses        742      745       +3     
- Partials      168      169       +1     

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