[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/build.rs

@@ -0,0 +1,42 @@
+// SPDX-FileCopyrightText: 2026 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+fn main() {
+    let base = "../../../external/ft8_lib";
+    let mut build = cc::Build::new();
+    build
+        .include(base)
+        .include(format!("{base}/common"))
+        .include(format!("{base}/fft"))
+        .include(format!("{base}/ft8"))
+        .define("_GNU_SOURCE", None)
+        .define("_POSIX_C_SOURCE", "200809L")
+        .file("src/ft8_wrapper.c")
+        .file(format!("{base}/common/monitor.c"))
+        .file(format!("{base}/fft/kiss_fft.c"))
+        .file(format!("{base}/fft/kiss_fftr.c"))
+        .file(format!("{base}/ft8/constants.c"))
+        .file(format!("{base}/ft8/crc.c"))
+        .file(format!("{base}/ft8/decode.c"))
+        .file(format!("{base}/ft8/ldpc.c"))
+        .file(format!("{base}/ft8/message.c"))
+        .file(format!("{base}/ft8/text.c"))
+        .flag_if_supported("-std=c99")
+        .flag_if_supported("-Wno-unused-const-variable")
+        .flag_if_supported("-Wno-unused-function")
+        .compile("trx_ft8");
+
+    println!("cargo:rustc-link-lib=m");
+
+    println!("cargo:rerun-if-changed=src/ft8_wrapper.c");
+    println!("cargo:rerun-if-changed={base}/common/monitor.c");
+    println!("cargo:rerun-if-changed={base}/fft/kiss_fft.c");
+    println!("cargo:rerun-if-changed={base}/fft/kiss_fftr.c");
+    println!("cargo:rerun-if-changed={base}/ft8/constants.c");
+    println!("cargo:rerun-if-changed={base}/ft8/crc.c");
+    println!("cargo:rerun-if-changed={base}/ft8/decode.c");
+    println!("cargo:rerun-if-changed={base}/ft8/ldpc.c");
+    println!("cargo:rerun-if-changed={base}/ft8/message.c");
+    println!("cargo:rerun-if-changed={base}/ft8/text.c");
+}