Add required-version field to configuration

[MatthewMckee4]

Summary

Closes #579. Adds a top-level required-version field to karva.toml and the [tool.karva] table in pyproject.toml. The value is a SemVer requirement; if the installed karva does not satisfy it, karva exits with a clear error before any tests run, which avoids the confusing failures that show up when CI or a teammate is on an older version that does not understand newer config keys.

required-version = ">=0.5.0"

The check runs at every config-loading entry point (karva.toml, --config-file / KARVA_CONFIG_FILE, and a discovered [tool.karva] table). Parsing uses semver::VersionReq, so an invalid specifier surfaces as a regular TOML parse error pointing at the offending value. Because required-version is not part of any profile, it is special-cased in the doc generator so the configuration reference still gets the section even though the metadata-driven walker only sees per-profile options.

Test Plan

ci

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing add-required-version (a5bc818) with main (eb3c800)}