Four bugs fixed:
1. persist.c: array job WAL loop aborted after index [0] when replication
returned -ENODEV (no standbys). Loop guard now uses the return value of
persist_wal_sync_append() which is 0 on local-disk success regardless of
replication status.
2. join.c: WAL-recovered worker slot (status=UNAVAIL) caused add_worker() to
return -EEXIST on reconnect. New EEXIST path in handle_worker_join() looks
up the existing slot by name, updates its endpoint, and clears UNAVAIL so
the normal hook/reply/on_worker_join flow proceeds.
3. xcectx.c: cbuf_get() in xce_hello() blocked indefinitely when the TCP
connection dropped because no error handler was registered. Added
handle_disconnect() which sets ctx->state=XCE_CTX_ERR and calls
cbuf_tryput(-ECONNRESET) to unblock the waiter, allowing
checking_connection() to detect the failure and rejoin.
4. cluster.c: persist_recover() was called before cluster_build_resources()
populated resmgr, causing restore_worker() → add_hostattrs_resvals() to
fail with EEXIST noise. Moved persist_recover() to after HOOK_AFTER_EVENT
so resmgr is fully populated before WAL replay.
