v1.9.1 — polish + clippy hygiene

Fixed

• benches/core_ops.rs's [[bench]] entry in Cargo.toml now declares required-features = ["fmt"], matching the existing declaration on transcendentals and conversions. The bench uses Decimal128::parse_str and failed to compile under cargo clippy --no-default-features --all-targets.
• src/ops/quantum.rs::tests::parse helper is #[cfg(feature = "fmt")] on the function itself rather than via runtime dispatch inside the body. Removes a "function never used" warning under --no-default-features --all-targets.
• tests/conformance.rs::parse_value documents the bare # null-test sentinel inline. An attempted fix mapping empty # to (NaN, INVALID) produced 28 status-mismatch failures because the runner's invoke() drops parse-time status; the fix is reverted, the comment now explains why the null-test category remains in the skip bucket and what would close it.

No public API change. Conformance numbers unchanged from 1.9.0 (8591 / 0 / 130).