chore: satisfy clippy on the current toolchain

[Eljees]

Lint fails on master itself with the clippy CI now installs, so it is red on every pull request opened since that bump - 13 of the 25 open ones when I checked, including 11 dependabot bumps, #1394, and my own #1515 and #1516. None of the findings are in code those branches touch, so nobody can turn their own PR green.

The five findings CI reports, and what this does with each:

lint	site	change
iter_kv_map	cobertura.rs:67	into_iter().filter_map(|(_, l)| ...) discards the key, so it is into_values()
collapsible_match	path_rewriting.rs:426	the two Some(..) arms fold into match guards
byte_char_slices x2	producer.rs:148	[b'T', b'N', b':'] is *b"TN:"
collapsible_match	producer.rs:112	allowed, see below

The last one is the only judgement call. The natural rewrite does not compile:

error[E0382]: use of moved value: `file`
   --> src/producer.rs:96:46

file is an Option<&mut impl Read> and so not Copy, and a value moved in a match guard counts as moved for the whole match, so the three content checks in handle_file have to stay in the arm bodies. I put an #[allow(clippy::collapsible_match)] on the function with that reason written next to it rather than restructure the match. Happy to do it differently if you would rather.

One thing I deliberately left out: an older clippy (1.95 here) additionally reports collapsible_match at src/parser.rs:742 - the current CI clippy does not. It cannot become a guard either, because the inner condition uses ?. I left parser.rs untouched so this does not conflict with #1515 and #1516, which both change that file. Say the word and I will fold it in.

Checked locally: cargo clippy --bins --tests --examples --all -- -D rust_2018_idioms -D warnings is clean apart from that one parser.rs line, cargo fmt -- --check passes, and cargo test --lib is unchanged at 133 passed (the four llvm_tools failures here need llvm-profdata and fail identically on master).

[Eljees]

Green now - 10/10, Lint included.

Two follow-up commits were needed, both from lints that only appear once the previous error stops the build early:

• redundant reference in panic! argument in tests/test.rs:584, which the library failing to compile had been hiding.
• Then, from my first attempt at that one: I inlined the binding as panic!("Failed to copy {gcda_zip_path:?}"), which is wrong here - the crate is edition = "2018", where implicit format-args capture does not exist, so the string is taken literally and rustc reports an unused placeholder. Dropping just the redundant & and keeping panic!("Failed to copy {:?}", gcda_zip_path) is what the lint actually wanted.

Worth flagging the reason for the churn: the clippy CI installs is newer than any toolchain I can reach, so I cannot reproduce its exact lint set locally and had to find those two through CI. If you have a preference for pinning the toolchain - a rust-toolchain.toml, or --locked clippy in the hook - that would stop this recurring on the next bump, and I am happy to send it separately.

Once this lands, #1515 and #1516 only need master merged in to go green; the same goes for the dependabot PRs.