[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.lock

@@ -2261,6 +2261,13 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "trx-aprs"
+version = "0.1.0"
+dependencies = [
+ "trx-core",
+]
+
 [[package]]
 name = "trx-backend"
 version = "0.1.0"
@@ -2415,6 +2422,7 @@ dependencies = [
  "toml",
  "tracing",
  "trx-app",
+ "trx-aprs",
  "trx-backend",
  "trx-core",
  "trx-ft8",