release: merge 2.1.2 hardening updates to master

[modestustr]

Summary

• Merge v2.1.2 hardening and documentation updates from development to master.
• Includes NuGet package README refresh commit.

Included commits

• release: hardening updates and v2.1.2 docs
• docs: refresh package readme for nuget
• related security/sample/auth/test updates already validated in development

Validation

• Local tests previously passed (net481 + net8.0).
• GitHub Actions on this PR will run Build & Test Suite and Security Pentest Suite.

Release note

• Package version remains 2.1.2; this PR aligns master with development before publish.