[refactor](decoders): consolidate decoder crates under src/decoders/

Move trx-ft8 and trx-wspr into src/decoders/ alongside a new trx-aprs
crate that extracts the Bell 202/AX.25 decoder from trx-server, giving
all three modems a consistent crate-per-decoder layout.

- src/decoders/trx-ft8/  (moved from src/trx-ft8/)
- src/decoders/trx-wspr/ (moved from src/trx-wspr/)
- src/decoders/trx-aprs/ (new — Bell 202 AFSK + AX.25/APRS decoder)
- trx-ft8/build.rs: fix external/ft8_lib relative path after move
- trx-server: drop decode::aprs module, use trx_aprs::AprsDecoder
- AprsPacket stays in trx-core (mirrors Ft8Message / WsprMessage)
- Workspace Cargo.toml updated with new member paths

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

Cargo.toml

@@ -4,8 +4,9 @@
 
 [workspace]
 members = [
-    "src/trx-ft8",
-    "src/trx-wspr",
+    "src/decoders/trx-aprs",
+    "src/decoders/trx-ft8",
+    "src/decoders/trx-wspr",
     "src/trx-core",
     "src/trx-protocol",
     "src/trx-app",