Fix HLC monotonicity, nil guards, and config parsing in S3 adapter

[Copilot]

Four correctness and safety issues flagged in review on the S3 adapter PR.

Changes

• kv/coordinator.go – dispatchTxn was not observing a caller-provided commitTS on the HLC, allowing subsequent clock.Next() calls to return timestamps smaller than a used commitTS. Added c.clock.Observe(commitTS) in the non-zero branch.

• adapter/s3.go – S3Server.Run() would panic if httpServer or listen were nil (possible when server is constructed without a listener). Added early nil-return errors for both fields.

• cmd/server/demo.go (coordinator nil check) – setupS3 accepted *kv.Coordinate and stored it into interfaces without a nil check. A nil pointer stored in an interface is non-nil and panics on method call. Added an explicit coordinator != nil guard.

• cmd/server/demo.go (raftS3MapStr parsing) – Used strings.Split(part, "=") which silently drops malformed entries and breaks on values containing =. Replaced with strings.SplitN(strings.TrimSpace(part), "=", 2) and returns an error for any entry not matching addr=s3addr.

// Before: silently skips bad entries
kv := strings.Split(part, "=")
if len(kv) == kvParts {
    leaderS3[raft.ServerAddress(kv[0])] = kv[1]
}

// After: returns error on malformed input, handles '=' in value
kv := strings.SplitN(strings.TrimSpace(part), "=", kvParts)
if len(kv) != kvParts {
    return nil, fmt.Errorf("invalid raft-s3 map entry %q: expected format addr=s3addr", part)
}
leaderS3[raft.ServerAddress(strings.TrimSpace(kv[0]))] = strings.TrimSpace(kv[1])

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