Discover standby for apply-wait read routing

[skhe]

Summary

• Resolve rwseparate=4 when the DM8 login response omits standby host/port. The driver now discovers a valid standby through primary metadata instead of dialing :0.
• Guard cleanup after failed standby metadata queries when no statement or rows were returned.
• Add a repeatable real-cluster verifier and document the tested scope.

Verification

• Local Orb DM8 ARM primary/standby/monitor: direct roles were PRIMARY, OPEN and STANDBY, OPEN with the same OGUID.
• ARM Linux dmPython: modes 0/1/4 routed a read-only query to primary/standby/standby with autocommit enabled and 0% primary reads.
• DECIMAL(30,8) written through mode 4 replicated to standby without losing fractional digits.
• Vendored Go tests passed; ordinary GB18030 real-database suite: 192 passed, 6 deselected.
• git diff --check and Python compile passed.

Scope limit

• HA verification currently runs locally because GitHub hosted runners cannot reach this Orb cluster. The GitHub CI matrix validates the code change against its existing single-node real-DM and build gates.
• A stop-primary trial did not prove automatic takeover: the monitor remained in confirmation mode and the standby stayed STANDBY, OPEN. This is recorded in the test report; no failover claim is made.