Fix private IP address filtering in DNS validation

[Copilot]

Private IP addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) were being incorrectly rejected by primary DNS servers because they're not in the China IP list used for geographic validation.

Changes

• Added isPrivateIP() helper to detect RFC 1918 IPv4 ranges and fc00::/7 IPv6 ULA
• Modified IsValidMsg() to skip geographic validation for private IPs - they're always valid regardless of DNS source
• Added test coverage for private IP detection and DNS message validation with private addresses

// Before: Private IPs from primary DNS would fail validation
// because ipRanger.Contains(10.0.0.1) returns false

// After: Private IPs bypass geographic checks
if isPrivateIP(ip) {
    up.logger.Printf("checkPrimary result %s: %s@%s -> private IP, skipping primary check", up.Address, domain, ip)
    continue
}

Internal network addresses are not subject to geographic routing restrictions and should always be accepted.

Original prompt

检查是否有逻辑丢弃了10.段的解析结果

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[naiba]

是解析10.x.x.x IP段 @copilot

[naiba]

我不是希望你过滤掉而是希望你不要过滤