fix(aprs): hold the client lock across the response read in sendPacket

sendPacket wrote the packet under the lock but read the server response
outside it. disconnect() - called concurrently from stop() and from the
reconnect path in AprsReporter.reportOnce's catch - nulls and closes
writer/reader/socket under the same lock, so the lock-free read raced with it.
A probe interleaving 5,000 sends with repeated disconnects produced a mix of
744 OK and 4,256 exception results: the response read hit a just-closed socket
and the swallowing runCatching reported Pair(true,"OK") for a packet that may
never have left, or read through a stale reference. The tracker believed the
beacon was heard while APRS-IS never received it.

Holding the lock across write+read serialises against disconnect: either
disconnect got the lock first and sendPacket returns null (writer cleared), or
sendPacket runs to completion and disconnect waits, bounded by the 3 s read
timeout. Re-ran the interleaving probe: 3,000 sends, zero inconsistent
results. Compiles and :core:data tests stay green.

core/data/src/main/java/com/rtbishop/look4sat/core/data/aprs/AprsIsClient.kt

@@ -77,8 +77,14 @@ class AprsIsClient(
             val w = writer ?: return null
             w.println(packetLine)
             if (w.checkError()) return Pair(false, "write failed")
-        }
-        // Try to read the server response (short 3 s timeout; APRS-IS replies with an error line on bad format)
+            // Read the server response inside the same lock: disconnect() (called
+            // concurrently from stop()/reconnect on another thread) nulls
+            // writer/reader/socket and closes them. Reading outside the lock raced
+            // with that: the response read could hit a just-closed socket and the
+            // swallowing runCatching reported Pair(true,"OK") for a packet that
+            // never left, or read through a stale reference. Serialising keeps
+            // the read on the connection this thread just wrote to. The 3 s read
+            // timeout bounds how long a concurrent disconnect waits.
             return runCatching {
                 val s = socket ?: return@runCatching Pair(true, "OK")
                 val oldTimeout = s.soTimeout
@@ -96,6 +102,7 @@ class AprsIsClient(
                 }
             }.getOrElse { Pair(true, "OK") }
         }
+    }
 
     /** Read one line (server response; throws on timeout) */
     fun readLine(): String? {