Move multithreaded input-range selection and overlap decisions into Rust using
an explicit scalar projection. C retains the round-buffer mutation, LDM
synchronization, prefix copying, and private job state, while Rust decides
whether a reusable section is available and where it begins.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --all-targets -- --test-threads=1
- cargo clippy --manifest-path rust/Cargo.toml --tests -- -D warnings
- make -B -C lib -j2 lib
- make -B -C tests -j2 test-cli-tests
- make -B -C tests -j2 test-rust-lib-smoke
ddidderr
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