fix(ci): clear RUSTFLAGS=-D warnings lints + cargo fmt sweep

[Laffinty]

CI was failing on commit 0ea6200 (M2-M6) with build errors on all three platforms. Two of the steps ('Build workspace' and 'Run unit tests') fired because RUSTFLAGS: -D warnings upgraded pre-existing warnings to errors in code that the author never compiled with -D warnings.

The third CI step ('Lint (clippy, deny warnings)') was never reached in the failed run, so it surfaced only when the build was fixed locally. Cleaning that up revealed the same class of issues across multiple crates — all stemming from the same M2-M6 commit.

This commit makes every CI step green:

Build / test fixes (the 3 errors the user originally hit):

• crates/idgen/src/lib.rs — drop 'mut' on 2 test variables
• sdk-rust/examples/batch.rs — drop unused ProbeInput, ProbeMode

Clippy -D warnings fixes (8 additional lints):

• crates/matchr/src/lib.rs — &[q.clone()] → slice::from_ref
• crates/probe/src/nmap_probes.rs — drop redundant 'as u16'
• crates/ouidb/src/lib.rs — drop redundant .into() on u8
• crates/datastore/src/lib.rs — len() > 0 → !is_empty()
• crates/branddb/src/lib.rs — to_ascii_lowercase() == → eq_ignore_ascii_case()
• crates/ffi-c/src/brand.rs — drop 2 redundant .into() to String;
  allow field_reassign_with_default on the
  function (the conditional branch logic
  makes struct-literal init awkward)
• crates/ffi-c/src/scan.rs — for i in 0..n → iter().take(n).enumerate()
• crates/ffi-c/src/types.rs — add # Safety section to read_c_str

Branddb OUI hex literal lints (preserved values, not blindly accepted):

• crates/branddb/src/builtin.rs — #[allow(mistyped_literal_suffixes)]
  on the RULES static + #[rustfmt::skip]
  to prevent re-bloat of the 5000-rule
  data table
• crates/branddb/src/yaml.rs — #[allow(unusual_byte_groupings)]
  on the oui_yaml_roundtrip test

The OUI lints are tricky: clippy thinks 0xC4_65_16 should be 0xC465_u16 (and similar), but accepting that suggestion would change the value (24-bit OUI prefix becoming a 16-bit one) and break vendor matching. Comments at the allow sites explain why.

Format check (cargo fmt --check):

• workspace-wide rustfmt sweep; most of the diff in crates/branddb/src/yaml.rs is a one-time alignment cleanup of the same OUI test fixtures.

Verified locally on windows-2022:

• RUSTFLAGS=-D warnings cargo test --workspace → 256 passed, 0 failed
• cargo clippy --workspace --all-targets -- -D warnings → clean
• cargo fmt --all -- --check → clean