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/peer/mod.rs

@@ -60,9 +60,9 @@ impl FromStr for ConnectionType {
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
-            "P" => Ok(ConnectionType::P),
-            "F" => Ok(ConnectionType::F),
-            "D" => Ok(ConnectionType::D),
+            "P" => Ok(Self::P),
+            "F" => Ok(Self::F),
+            "D" => Ok(Self::D),
             _ => Err(ParseConnectionTypeError),
         }
     }
@@ -71,11 +71,11 @@ impl FromStr for ConnectionType {
 impl fmt::Display for ConnectionType {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let s = match self {
-            ConnectionType::P => "P",
-            ConnectionType::F => "F",
-            ConnectionType::D => "D",
+            Self::P => "P",
+            Self::F => "F",
+            Self::D => "D",
         };
-        write!(f, "{}", s)
+        write!(f, "{s}")
     }
 }
 
@@ -93,7 +93,8 @@ pub struct Peer {
 }
 impl Peer {
     #[allow(clippy::too_many_arguments, dead_code)]
-    pub fn new(
+    #[must_use]
+    pub const fn new(
         username: String,
         connection_type: ConnectionType,
         host: String,