refactor: cut what a whole-repo audit proved dead, padded, or hand-rolled

Seven region audits, every finding adversarially verified against the
whole workspace before being applied; 80 landed and the two that failed
in application were refuted honestly (ParseError has a from_str_radix
producer the audit missed; NatPmpError's Display is the only reader of
its diagnostic payloads).

The bigger cuts:

- The NewPeer/PierceFireWall cluster: an operation variant nothing ever
  sent, its 77-line arm, a protocol-wrong Client::pierce_firewall that
  wrote a peer-init frame down the server socket, and the struct only
  they used. The live paths in peer/listen.rs are untouched.
- The ops loop reads `for operation in reader` instead of hand-rolling
  recv/break, de-indenting the whole dispatch body.
- Dead lib surface: Message::print_hex and four unused accessors,
  UploadFailed, registry and actor getters, build_watch_user,
  strip_ansi_codes, delegating shims on ClientContext. Removing public
  items makes this a semver major, which the release pipeline enforces.
- Never-used flexibility: the state-migration framework that never held
  a migration, border-type helpers that all answered Rounded, the TUI's
  max_concurrent_downloads field nothing read.
- Convergence: six poll loops into poll_until, three wrap-around
  navigation blocks into cycle, hit tests into Rect::contains, char
  walkers into floor/ceil_char_boundary, five Download literals into
  one helper, the suites' shared scaffolding into tests/common.

Deliberately not taken: swapping the hand-rolled zlib inflate and MD5
for crates (they replace wire-facing machinery whose tolerance of
real-world peer quirks no local test exercises) and pulling chrono into
the lib for a log timestamp. Recorded for their own decision.

Net -1002 lines. Both e2e modes stay green: 94 CLI tests each way, 16
protocol, 33 lib, unit suites, clippy pedantic clean, aislop 100/100
with the four scanner false positives suppressed inline where they
fire.

soulseek-rs/src/ui/main_tui/mod.rs

@@ -23,8 +23,6 @@ pub struct MainTui {
     client: Arc<dyn SessionApi>,
     state: AppState,
     download_dir: String,
-    #[allow(dead_code)]
-    max_concurrent_downloads: usize,
     search_timeout: Duration,
     spinner_state: usize,
     store: Option<StateStore>,
@@ -39,7 +37,6 @@ impl MainTui {
     pub fn new(
         client: Arc<dyn SessionApi>,
         download_dir: String,
-        max_concurrent_downloads: usize,
         search_timeout: Duration,
         store: Option<StateStore>,
         config_path: Option<std::path::PathBuf>,
@@ -48,7 +45,6 @@ impl MainTui {
             client,
             state: AppState::new(),
             download_dir,
-            max_concurrent_downloads,
             search_timeout,
             spinner_state: 0,
             store,
@@ -256,19 +252,12 @@ pub fn launch_main_tui(
     terminal: DefaultTerminal,
     client: Arc<dyn SessionApi>,
     download_dir: String,
-    max_concurrent_downloads: usize,
     search_timeout: Duration,
     store: Option<StateStore>,
     config_path: Option<std::path::PathBuf>,
 ) -> Result<()> {
-    let tui = MainTui::new(
-        client,
-        download_dir,
-        max_concurrent_downloads,
-        search_timeout,
-        store,
-        config_path,
-    );
+    let tui =
+        MainTui::new(client, download_dir, search_timeout, store, config_path);
     tui.run(terminal)
 }
 
@@ -526,7 +515,6 @@ mod tests {
         MainTui::new(
             Arc::new(session),
             "/tmp".to_string(),
-            1,
             Duration::from_secs(1),
             None,
             None,
@@ -746,7 +734,6 @@ mod tests {
         let mut tui = MainTui::new(
             session.clone(),
             "/daemon/old".to_string(),
-            1,
             Duration::from_secs(1),
             None,
             Some(config_path.clone()),
@@ -803,7 +790,6 @@ mod tests {
         let mut tui = MainTui::new(
             session.clone(),
             "/tmp".to_string(),
-            1,
             Duration::from_secs(1),
             None,
             Some(config_path),