Fix clippy warnings and a stale test assertion surfaced by check.yml

check.yml runs clippy -D warnings and cargo test for the first time in
this repo's history, surfacing pre-existing lint debt across all four
workspace crates plus one stale test assertion (breadpad-shared's
Palette default-background test still expected bread-theme's old
#1e1e2e; the upstream design system moved to #0c0c0c a while back and
nothing caught the drift since tests never ran in CI).

All fixes are mechanical: needless returns/closures/borrows, manual
Default impls replaceable by #[derive], map_or -> is_some_and, manual
range checks -> RangeInclusive::contains, a non-idiomatic sort_by ->
sort_by_key, an overly complex inline type given an alias, and moving
a function that ended up defined after its own test module. The one
exception is NoteType::from_str, which clippy flags for shadowing
std::str::FromStr's name — left as `#[allow(...)]` with a comment
rather than renamed, since it has 30+ call sites across every crate
in the workspace and returns Self directly rather than Result.

breadpad-test/src/main.rs

@@ -389,7 +389,7 @@ fn cmd_show(index: usize, corpus_path: &Path, tier: &TierArg) -> Result<()> {
     println!();
 
     let sep = "─".repeat(62);
-    println!("{:<14} {:<26} {}", "field", "expected", "actual");
+    println!("{:<14} {:<26} actual", "field", "expected");
     println!("{sep}");
     println!(
         "{:<14} {:<26} {}",