`src/vfs.c:1271: vfsMainFileClose: Assertion '(f->database->shm.lock[0] == 0)'`

In the LXD test suite, during the `clustering_handover`, an assertion failure in `dqlite` [was found](https://github.com/simondeziel/lxd/actions/runs/18391256026/job/52401997054#step:17:32583):

```
2025-10-09T23:08:03.9744500Z + lxd shutdown
2025-10-09T23:08:03.9761816Z time="2025-10-09T23:08:03Z" level=info msg="Found cluster member whose role needs to be changed" candidateAddress="100.64.1.103:8443" local="100.64.1.101:8443" newRole=spare
2025-10-09T23:08:03.9765596Z time="2025-10-09T23:08:03Z" level=info msg="Demoting offline member during rebalance" candidateAddress="100.64.1.103:8443"
2025-10-09T23:08:03.9890740Z time="2025-10-09T23:08:03Z" level=info msg="Completed heartbeat round" duration=142.222769ms local="100.64.1.101:8443"
2025-10-09T23:08:03.9943000Z time="2025-10-09T23:08:03Z" level=info msg="Starting heartbeat round" local="100.64.1.101:8443" mode=initial
2025-10-09T23:08:03.9952887Z time="2025-10-09T23:08:03Z" level=warning msg="Failed heartbeat" err="Failed to send heartbeat request: Put \"https://100.64.1.103:8443/internal/database\": dial tcp 100.64.1.103:8443: connect: connection refused" remote="100.64.1.103:8443"
2025-10-09T23:08:04.0008186Z time="2025-10-09T23:08:04Z" level=info msg="Partial heartbeat received" local="100.64.1.104:8443"
2025-10-09T23:08:04.0011674Z time="2025-10-09T23:08:04Z" level=info msg="Partial heartbeat received" local="100.64.1.102:8443"
2025-10-09T23:08:04.0025965Z time="2025-10-09T23:08:04Z" level=info msg="Asked to shutdown by API" force=false
2025-10-09T23:08:04.0029878Z time="2025-10-09T23:08:04Z" level=info msg="Starting shutdown sequence" signal="power failure"
2025-10-09T23:08:04.0031115Z time="2025-10-09T23:08:04Z" level=info msg="Unwatching path" driver=inotify path=/var/lib/ubuntu-advantage/interfaces/lxd-config.json
2025-10-09T23:08:04.0032123Z time="2025-10-09T23:08:04Z" level=info msg="Transferring leadership" address="100.64.1.101:8443"
2025-10-09T23:08:04.0050157Z time="2025-10-09T23:08:04Z" level=warning msg="Failed heartbeat" err="Failed to send heartbeat request: Put \"https://100.64.1.103:8443/internal/database\": dial tcp 100.64.1.103:8443: connect: connection refused" remote="100.64.1.103:8443"
2025-10-09T23:08:04.0089964Z lxd: src/vfs.c:1271: vfsMainFileClose: Assertion `(f->database->shm.lock[0] == 0)' failed.
2025-10-09T23:08:04.0119308Z time="2025-10-09T23:08:04Z" level=warning msg="Failed adding member event listener client" err="dial tcp 100.64.1.103:8443: connect: connection refused" local="100.64.1.104:8443" remote="100.64.1.103:8443"
2025-10-09T23:08:04.0123219Z time="2025-10-09T23:08:04Z" level=warning msg="Failed adding member event listener client" err="dial tcp 100.64.1.103:8443: connect: connection refused" local="100.64.1.102:8443" remote="100.64.1.103:8443"
2025-10-09T23:08:04.0125165Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite connected outbound" local="100.64.1.102:40880" name=raft remote="100.64.1.104:8443"
2025-10-09T23:08:04.0140110Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite proxy started" local="100.64.1.104:8443" name=dqlite remote="100.64.1.1:40880"
2025-10-09T23:08:04.0143688Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite proxy started" local="100.64.1.102:40880" name=raft remote="100.64.1.104:8443"
2025-10-09T23:08:04.0172181Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite connected outbound" local="100.64.1.104:33382" name=raft remote="100.64.1.102:8443"
2025-10-09T23:08:04.0187323Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite proxy started" local="100.64.1.102:8443" name=dqlite remote="100.64.1.1:33382"
2025-10-09T23:08:04.0191223Z time="2025-10-09T23:08:04Z" level=info msg="Dqlite proxy started" local="100.64.1.104:33382" name=raft remote="100.64.1.102:8443"
2025-10-09T23:08:05.2390792Z Error: Put "http://unix.socket/internal/shutdown?force=false": EOF
2025-10-09T23:08:05.2392846Z time="2025-10-09T23:08:05Z" level=info msg="Dqlite proxy stopped" local="100.64.1.104:8443" name=dqlite remote="100.64.1.1:52886"
2025-10-09T23:08:05.2394393Z time="2025-10-09T23:08:05Z" level=info msg="Dqlite proxy stopped" local="100.64.1.102:8443" name=dqlite remote="100.64.1.1:40832"
2025-10-09T23:08:05.2395726Z time="2025-10-09T23:08:05Z" level=warning msg="Dqlite proxy failed" err="first: remote -> local: read tcp 100.64.1.102:42778->100.64.1.101:8443: read: connection reset by peer" local="100.64.1.102:42778" name=raft remote="100.64.1.101:8443"
2025-10-09T23:08:05.2396937Z time="2025-10-09T23:08:05Z" level=info msg="Dqlite proxy stopped" local="100.64.1.102:42778" name=raft remote="100.64.1.101:8443"
2025-10-09T23:08:05.2397788Z time="2025-10-09T23:08:05Z" level=info msg="Dqlite proxy stopped" local="100.64.1.104:33280" name=raft remote="100.64.1.101:8443"
2025-10-09T23:08:05.2404614Z suites/clustering.sh: line 2820: 32574 Aborted (core dumped) nsenter -n -m -t "${pid}" lxd --logfile "${LXD_DIR}/lxd.log" "${SERVER_DEBUG-}" "$@" 2>&1
2025-10-09T23:08:05.2405206Z + cleanup
```

There are many failures in the other tests ran with that PR but they are unrelated to this rare `dqlite` issue.

Start with the assertion at src/vfs.c:1271 and the clustering_handover failure in the LXD test suite. Trace why the shared-memory lock remains set during vfsMainFileClose, then reproduce the rare shutdown failure and verify that the assertion no longer occurs.

Written by the indexing model from the issue text.