ci: enforce coverage and import quality gates

[KageRyo]

Closes #54

• enforce an 80% total coverage floor in pytest and CI
• treat Pyright missing imports as errors
• document the 80% baseline and dependency/interpreter expectations
• use uv for source development environment setup

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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