Resolve merge conflicts for codecov integration

[Copilot]

Merged latest main branch into codecov PR to resolve conflicts. Main branch migrated from Poetry to uv/hatchling build system between initial codecov implementation and now.

Changes

• CI workflow - Updated pytest command to uv run pytest --cov-branch --cov-report=xml and added Codecov upload step
• Dependencies - Added pytest-cov>=6.0.0,<7.0.0 to [project.optional-dependencies] test group in pyproject.toml
• README - Added codecov badge for coverage visibility
• Build system - Adapted codecov integration from Poetry to hatchling/uv ecosystem

All other conflicts resolved by accepting main branch changes (formatting, type hints, new features unrelated to coverage).

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