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-lib/src/message/server/message_factory.rs

@@ -137,14 +137,6 @@ impl MessageFactory {
             .write_int32(port.into())
             .clone()
     }
-    #[must_use]
-    pub fn build_watch_user(token: u32) -> Message {
-        Message::new()
-            .write_raw_bytes([5, 0, 0, 0, 0].to_vec())
-            .write_int32(token)
-            .clone()
-    }
-
     /// Ask the server (code 7) for a user's online status.
     #[must_use]
     pub fn build_get_user_status(username: &str) -> Message {
@@ -280,15 +272,6 @@ fn test_build_get_user_stats() {
     assert_eq!(expect, message.get_data());
 }
 
-#[test]
-fn test_build_watch_user() {
-    let token: u32 = 223;
-    let message = MessageFactory::build_watch_user(token);
-    let expect: Vec<u8> = [5, 0, 0, 0, 0, 223, 0, 0, 0].to_vec();
-
-    assert_eq!(expect, message.get_data());
-}
-
 #[test]
 fn test_build_login_message() {
     let message = MessageFactory::build_login_message(