test: live persistent-spool restart end-to-end

Adds a docker-compose service `server-spool` that runs secs_server
with --spool-dir pointed at a named volume.  Two-phase Python
harness (interop/spool_persistence_test.py):

  1. Enqueue phase: force-spool one S6F11(CEID=300) via the
     SPOOL_ON / START / SPOOL_OFF RCMD trio, then disconnect.
  2. Driver runs `docker compose restart server-spool` between
     the phases — the named volume preserves the journal files.
  3. Drain phase: reconnect, send S6F23(Transmit), verify the
     replayed S6F11 carries CEID 300.

Surfaces a real interop bug along the way: secsgem-py 0.3.0 encodes
RSDC (and other "single-byte status" fields) as <U1>, while SEMI E5
spells them as <B>.  Our `as_binary_first` was strict on Binary; now
accepts either (the byte semantics are identical, and the leniency is
symmetric with the U-type widening from the first interop commit).

Result: enqueue → docker restart → drain returns CEID 300 cleanly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -75,8 +75,31 @@ services:
     command: ["python3", "/app/interop/passive_equipment.py", "--port", "5000"]
     networks: [secs]
 
+  # Server variant for the persistent-spool restart test.  Identical to
+  # `server` but with --spool-dir pointed at a named volume so spooled
+  # messages survive `docker compose restart server-spool`.
+  server-spool:
+    <<: *base
+    depends_on:
+      builder:
+        condition: service_completed_successfully
+    command:
+      - /app/build/secs_server
+      - --port
+      - "5000"
+      - --device
+      - "0"
+      - --spool-dir
+      - /spool
+    volumes:
+      - .:/app
+      - build:/app/build
+      - spool:/spool
+    networks: [secs]
+
 networks:
   secs: {}
 
 volumes:
   build: {}
+  spool: {}