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/client/uploads.rs

@@ -47,8 +47,6 @@ impl Client {
             .and_then(|ctx| ctx.own_privileges)
     }
 
-    /// How many uploads run at once. Lowering it does not interrupt transfers
-    /// already in flight; the queue simply refills more slowly.
     /// Queued-upload states recorded since the last call, for a caller that
     /// samples [`Client::uploads`] and would otherwise miss a peer that queued
     /// and was served between two samples.
@@ -60,6 +58,8 @@ impl Client {
             .unwrap_or_default()
     }
 
+    /// How many uploads run at once. Lowering it does not interrupt transfers
+    /// already in flight; the queue simply refills more slowly.
     pub fn set_upload_slots(&self, slots: usize) {
         if let Ok(mut ctx) = self.context.write_safe() {
             ctx.upload_slots = slots.max(1);