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>
