fix(core): restore exact single-query index throughput

[seonghobae]

Summary

• restore the dedicated parallel single-query exact scoring path after the batch-kernel refactor
• retain the shared exact reduction/report helpers and batch contract unchanged
• add explicit packed-batch validation coverage

Measured reason

On the 6,578 x 3,072 synthetic/non-identifying production-shape snapshot, routing one query through the generic batch fold made the first accepted full Lineage read 67.922 ms and warm reads 12.863-15.751 ms. The restored owner path removes that demonstrated regression without changing authorization, scores, ordering, or digests.

Validation

• cargo +1.97.1 fmt --all -- --check
• cargo +1.97.1 clippy --workspace --all-targets -- -D warnings
• cargo +1.97.1 llvm-cov --package rankweave-core --fail-under-lines 100 --fail-under-functions 100 --fail-under-regions 100 (23 passed; 100% line/function/region)
• PYTHONPATH=. uvx uv@0.12.1 run --extra dev pytest -q (692 passed)

This is a regression repair on the #41 stack. It does not claim the general four-distinct-query 20 ms SLO; that remains under exact accelerator/full-path measurement.

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[devin-ai-integration]

Devin Review found 1 potential issue.