[refactor](decoders): extract CW decoder into trx-cw crate

Move the Goertzel-based CW decoder out of trx-server::decode::cw into
a dedicated src/decoders/trx-cw crate, matching the layout of trx-aprs,
trx-ft8, and trx-wspr. The decode module is now empty and removed.

- src/decoders/trx-cw/ (new — Goertzel + Morse decoder)
- trx-server: drop decode module entirely, use trx_cw::CwDecoder
- CwEvent stays in trx-core (mirrors AprsPacket / Ft8Message / WsprMessage)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -2336,6 +2336,13 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "trx-cw"
+version = "0.1.0"
+dependencies = [
+ "trx-core",
+]
+
 [[package]]
 name = "trx-frontend"
 version = "0.1.0"
@@ -2425,6 +2432,7 @@ dependencies = [
  "trx-aprs",
  "trx-backend",
  "trx-core",
+ "trx-cw",
  "trx-ft8",
  "trx-protocol",
  "trx-wspr",