ci: enforce coverage thresholds

[guiPython]

Summary

• Add a Codecov policy requiring at least 95% project coverage.
• Require 100% coverage for changed lines with no tolerance.
• Document the CI coverage gate in the README and project standards.

Issue: N/A

Type Of Change

• Bug fix
• Feature
• Documentation
• Tests
• Benchmarks or performance
• Refactor or maintenance

API And Behavior

• No public API or documented behavior changed.
• Public API or documented behavior changed, and docs/examples were updated.
• This is a breaking change and the migration impact is explained below.

Details: This change configures repository coverage policy only.

Testing

• go test ./...: no Go source changed
• Targeted tests: parsed codecov.yml locally
• Coverage check, if applicable: Codecov validation endpoint returned Valid!
• Race test, if applicable: no Go source changed
• make lint, if public APIs/docs/broad behavior changed: no Go source or public API changed

Paste relevant output or explain why a check was not run:

codecov YAML: valid
Codecov validation: Valid!
project target: 95.0, threshold: 0.0
patch target: 100.0, threshold: 0.0
git diff --check: passed

Risk

• Coverage regressions can block merges once the Codecov project check is required.
• The thresholds were validated by Codecov and match the documented policy.

Checklist

• I followed CONTRIBUTING.md and checked the relevant project standards when applicable.
• The diff is focused and excludes unrelated local edits.
• Behavior changes include tests.
• Public behavior changes include docs, examples, or README guidance.
• Performance-sensitive changes include benchmarks or an explanation.
• Removal paths clear discarded references where relevant.

Notes For Reviewers

• Core internal data-structure packages retain their existing 100% coverage guideline.
• After codecov/project appears and passes, the main ruleset will require that check instead of the generic upload job.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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