[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>

src/decoders/trx-ft8/Cargo.toml

@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: 2026 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-ft8"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+libc = "0.2"
+
+[build-dependencies]
+cc = "1"