Migrate constant-size chunks_exact to as_chunks for the Rust 1.98 clippy lint

[ciaranra]

Rust 1.98 reached CI (which floats latest stable) and its new clippy::chunks_exact_to_as_chunks lint fails rust-lint on every open PR against pre-existing code. This migrates all 91 constant-size chunks_exact call sites across 20 files to as_chunks, mostly via cargo clippy --fix plus four manual sites in pecos-rslib (where the array chunks also simplify from_le_bytes conversions by removing intermediate buffer copies).

No behavior change: as_chunks::<N>().0 yields exactly the complete chunks that chunks_exact(N) yielded, as arrays instead of slices.

Verification: both CI clippy lanes reproduced locally on Rust 1.98.0 (--workspace --all-targets --all-features -- -D warnings, and the per-crate no-LLVM lane including pecos with --no-default-features), cargo fmt --all -- --check clean, and lib tests pass for the touched crates (pecos-engines, pecos-simulators, pecos-qec, pecos-foreign, pecos-neo). The pecos-rslib test binary does not link in this local environment (missing libpython at link time, unrelated to this change); its code compiles clean under clippy and CI will run its tests.