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/types.rs

@@ -109,7 +109,8 @@ pub struct Download {
 }
 
 impl Download {
-    pub fn is_finished(&self) -> bool {
+    #[must_use]
+    pub const fn is_finished(&self) -> bool {
         matches!(
             self.status,
             DownloadStatus::Completed
@@ -118,7 +119,8 @@ impl Download {
         )
     }
 
-    pub fn bytes_downloaded(&self) -> u64 {
+    #[must_use]
+    pub const fn bytes_downloaded(&self) -> u64 {
         match &self.status {
             DownloadStatus::InProgress {
                 bytes_downloaded, ..
@@ -131,7 +133,8 @@ impl Download {
         }
     }
 
-    pub fn speed_bytes_per_sec(&self) -> f64 {
+    #[must_use]
+    pub const fn speed_bytes_per_sec(&self) -> f64 {
         match &self.status {
             DownloadStatus::InProgress {
                 speed_bytes_per_sec,