Volatile fields with inconsistent synchronization in IpcClient

**Affected version:** HEAD

**File:** `maven-resolver-named-locks-ipc/src/main/java/org/eclipse/aether/named/ipc/IpcClient.java:78-87`

Fields `initialized`, `socket`, `output`, `input`, `receiver` are `volatile` but their compound use (read + method call) lacks synchronization. `send()` reads `output` into a local variable at line 316 without holding the monitor, then synchronizes on the local reference at line 323, but `close()` nulls `output` under `synchronized(this)` at line 361. `receive()` has the same pattern with `input`.

Originally reported in #1944.

Start in maven-resolver-named-locks-ipc/src/main/java/org/eclipse/aether/named/ipc/IpcClient.java, reviewing fields at lines 78-87 and the send(), receive(), and close() synchronization paths around lines 316, 323, and 361. Confirm that compound access to output and input is synchronized consistently with close(), with no unsynchronized read-and-method-call path remaining.

Written by the indexing model from the issue text.