style: crank clippy to pedantic+nursery and fix all lints

Enable the full clippy all + pedantic + nursery groups as workspace warnings,
with a small documented allow-list for the pedantic/nursery lints this project
intentionally does not follow (rustdoc-style requirements, deliberate numeric
casts in the protocol/UI layers, and a few noisy nursery lints). Also enable
the rust unused_lifetimes and unsafe_op_in_unsafe_fn lints.

Fixed every resulting warning (527 -> 0) mechanically per clippy's suggestions:
inline format args, const fns, item ordering, let-else, redundant clones,
map_or, cast_lossless, Eq derives, associated-fn conversions for methods that
never used self, and removing unnecessary Result wrappers whose call sites could
never propagate an error. No runtime behavior changed; cargo fmt applied.

cargo clippy --workspace --all-targets: 0 warnings. All tests pass.

soulseek-rs-lib/src/message/handlers.rs

@@ -18,6 +18,7 @@ impl<Op> Default for Handlers<Op> {
 }
 
 impl<Op> Handlers<Op> {
+    #[must_use]
     pub fn new() -> Self {
         Self {
             handlers: HashMap::new(),
@@ -31,6 +32,7 @@ impl<Op> Handlers<Op> {
         self.handlers.insert(handler.get_code(), Box::new(handler));
         self
     }
+    #[must_use]
     pub fn get_handler(
         &self,
         code: u8,