Add show-config command to print resolved configuration

[MatthewMckee4]

Summary

Closes #763. Adds a karva show-config subcommand that resolves the same Settings the test runner builds — defaults layered with karva.toml/pyproject.toml and the selected profile — and prints them as TOML. This gives users a way to ask "what config is karva actually running with?" when debugging precedence between built-in defaults, [profile.default], a named profile, and CLI overrides.

The command takes the same --config-file and --profile/-P flags as karva test, so the resolution path matches what the runner would use. Output is a flat TOML body of the merged option groups (src, terminal, test, coverage) with every field populated to make defaults explicit.

karva show-config              # default profile
karva show-config --profile ci

The round-trip from resolved ProjectSettings back to Options lives on ProjectSettings::to_options() so there is a single source of truth (defaults still flow through to_settings). Runtime-only fields (filter, run_ignored, coverage disabled) are excluded since they don't come from configuration files, and fail_fast is omitted because max_fail is the canonical form. A future --source flag, as mentioned in the issue, would build on the same plumbing to annotate each value with its origin.

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-show-config (d125c10) with main (eb3c800)}