[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.toml

@@ -5,6 +5,7 @@
 [workspace]
 members = [
     "src/decoders/trx-aprs",
+    "src/decoders/trx-cw",
     "src/decoders/trx-ft8",
     "src/decoders/trx-wspr",
     "src/trx-core",