perf: update reverse ends aggregation boundary

[samukweku]

Closes the pyjanitor side of janitor-rs issue #94.

Updates reverse ends+matches aggregation dispatch for the length-free Rust API and includes a narrow fallback for older installed janitor_rs releases that still require length. The ordinary reverse ends path keeps its existing length argument.

Tests: pixi run pytest tests/functions/test_conditional_join.py -k agg_rev -q (20 passed).

Benchmark: the corresponding Rust PRs compare old maps vs compact slots across tiny, large, very large, and super large duplicate/unique workloads; results are documented on janitor-rs issue #94.

[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.