Exclude LAG member ports from excessive tagged VLANs audit

[tvancott42]

Summary

• LAG child ports no longer trigger false positive "Excessive Tagged VLANs" issues. LAG member ports have a raw forward: "all" config that is irrelevant since they're assimilated into the parent LAG port. Ports with both lag_idx (number) and aggregated_by (number) are now marked as IsLagChild and skipped by most audit rules.
• Unused port detection still applies to LAG child ports. Rules can opt in to evaluate LAG children via AppliesToLagChildPorts. Currently only UnusedPortRule opts in, so disconnected LAG member ports are still flagged if not disabled.

Closes #206

Test plan

• 10 new tests covering LAG detection, parent vs child, multiple members, missing fields, and integration with both AccessPortVlanRule (excluded) and UnusedPortRule (included)
• All 5,138 existing tests pass, 0 build warnings
• Run audit on NAS deployment and verify LAG ports no longer generate excessive VLAN issues