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/port_mapping/nat_pmp.rs

@@ -52,8 +52,6 @@ impl From<io::Error> for NatPmpError {
 /// a shorter lifetime than requested, so callers must use these values.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct MapResponse {
-    /// Seconds since the gateway's port table was initialised (reboot detector).
-    pub epoch: u32,
     pub internal_port: u16,
     pub external_port: u16,
     pub lifetime: u32,
@@ -87,12 +85,6 @@ pub fn map_tcp(
         return Err(NatPmpError::ShortResponse);
     }
     Ok(MapResponse {
-        epoch: u32::from_be_bytes([
-            response[4],
-            response[5],
-            response[6],
-            response[7],
-        ]),
         internal_port: u16::from_be_bytes([response[8], response[9]]),
         external_port: u16::from_be_bytes([response[10], response[11]]),
         lifetime: u32::from_be_bytes([