ci: add macOS unit test coverage

[OpenCow42]

Summary

• narrow the shared potassiumProvider scheme to the unit-test target
• extend the existing macOS job with an unsigned xcodebuild test step
• use a CI-only macOS 26.4 deployment-target override for the hosted runner while leaving the project target at 26.5
• update testing documentation to distinguish unit and UI test paths

Validation

• shared scheme XML and workflow YAML parse successfully
• unsigned macOS build succeeded locally after rebasing onto current main
• GitHub Actions macOS build and unit-test steps passed on run 30158977756
• CI log reports TEST SUCCEEDED and does not build or run potassiumProviderUITests
• git diff --check passes

Closes #7