feat(core): add persistent exact semantic index

[seonghobae]

Summary

• add an immutable, atomically replaceable exact semantic-unit index owned by Rust
• bind snapshots and results to model, dimension, vector, input, and output digests
• require caller-supplied authorized candidate identities and support digest-equivalent packed identity transport

This is a stacked prerequisite on #41. It does not by itself prove LineageWeave’s 20 ms full-path SLO.

Validation

• cargo +1.97.1 llvm-cov --package rankweave-core --fail-under-lines 100 --fail-under-functions 100 --fail-under-regions 100 (21 passed; 100%)
• cargo +1.97.1 clippy --workspace --all-targets --all-features -- -D warnings
• PYTHONPATH=. uvx uv@0.12.1 run --frozen --extra dev pytest (688 passed)
• macOS cp310-abi3 wheel and sdist archive verification
• synthetic 6,578 x 3,072 benchmark: exact rank 18.249 / 9.083 / 5.023 ms at 1 / 2 / 4 workers, identical digests; cold build 336.780 / 317.419 / 306.615 ms

Signed-off-by: Seongho Bae seongho.bae@gmail.com

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