Add --partition slice:M/N for round-robin test slicing

[MatthewMckee4]

Summary

Adds a --partition slice:M/N flag to karva test for splitting a test run across CI jobs. Tests are sorted by qualified name and distributed round-robin: test 1 to slice 1, test 2 to slice 2, ..., test N+1 to slice 1, and so on. Running every slice:1/N through slice:N/N together covers every collected test exactly once.

karva test --partition slice:1/3
karva test --partition slice:2/3
karva test --partition slice:3/3

The slice is applied inside partition_collected_tests after the --last-failed filter and before bin-packing across workers, so it composes with both. Slice membership shifts when the test set changes, which is the documented tradeoff against a hash-based scheme — the upside is no historical data is required.

Closes #587.

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 matt/partition-slice (4c9d712) with main (56b23da)}