perf: remove redundant explicit reverse range length

[samukweku]

Summary

Closes the pyjanitor side of janitor-rs#107 and contributes to #23.

The Rust explicit reverse starts/ends kernels now derive their own bounded capacity hint and no longer accept a redundant length argument. This change removes the corresponding Python-side ends.max() - starts.min() computation and stops passing length to the size kernel.

ELI5: the Python layer was calculating a size estimate that Rust could calculate more safely from the actual ranges. Removing it keeps one source of truth and avoids reserving based on a potentially misleading span, especially with duplicate labels.

Performance and memory

The paired janitor-rs PR includes Criterion comparisons against the old HashMap-per-label implementation for unique and duplicate labels at n=32, 10,000, 100,000, and 1,000,000. The compact kernels are generally ~1.5–3x faster for min/max/prod/size and ~15–40% faster for sum, with broadly comparable memory and substantially lower peak live memory in representative duplicate-label cases. The benchmark records allocation count, allocated bytes, and peak live memory.

Verification

• Python syntax checks passed.
• Pre-commit checks passed except pixi-install, which was skipped because dependency resolution was unavailable due to a transient DNS failure; ruff, formatting, pydoclint, interrogate, whitespace, and file checks passed.

Coordinated janitor-rs PR: branch issue-107-explicit-ranges.

[github-actions]

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[samukweku]

Contract documentation is now included in this PR and its branch AGENTS.md.

• Direct Rust reverse-match kernels reject an empty flattened matches tape; malformed tape shape is not silently accepted.
• A batch with no candidates at all is valid at the Python level, so pyjanitor pre-filters that case and returns the normal typed empty result before dispatching to Rust.
• Individual zero-width ranges remain valid when the overall tape is non-empty.
• The comparison/producer layer owns the matches values-in-{0,1} invariant; the Rust boundary validates tape shape.
• The compatibility length argument exists only for older janitor-rs wheels.

An ELI5 explanation is included in the Python module docstring and AGENTS.md.