clippy: clean up all warnings

Mostly `cargo clippy --fix`: needless borrows, redundant field names,
match -> if let, io::Error::other, Option::filter/map, let chains, c""
literals. By hand: drop unused impl lifetimes, a type alias for the
font preload map, Iterator::find in next_buffer, contains_key, and
allow new_ret_no_self where constructors intentionally return the
boxed/generic wrapper type.

No functional change; tests and release build unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/fonts.rs

@@ -70,10 +70,13 @@ impl PartialEq for ComparableF32 {
 
 impl Eq for ComparableF32 {}
 
+/// font name -> (font name, [(size, glyphs to preload)])
+type RequiredFonts = HashMap<&'static str, (&'static str, Vec<(f32, &'static str)>)>;
+
 pub struct FontMap {
     fonts: HashMap<(&'static str, ComparableF32), draw::Font<'static>>,
     font_paths: HashMap<&'static str, String>,
-    required_fonts: HashMap<&'static str, (&'static str, Vec<(f32, &'static str)>)>,
+    required_fonts: RequiredFonts,
 }
 
 impl FontMap {