security-group: compare IPv6 ranges numerically

[tllovesxs]

Summary

• Replace lexicographical IPv6 range ordering with numeric address comparison.
• Preserve the existing IPv4 validation path.

Fixes #30.

Test plan

• git diff --check
• Verify that 2001:db8::2-2001:db8::10 is accepted by numeric range validation.
• Verify that reversed IPv6 ranges are rejected.
• The full security-group integration suite requires the project test environment and was not run locally.

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[zsphere-opensource]

Could you add regression tests for this validation path?

Please cover at least:

• valid range: 2001:db8::2-2001:db8::10
• reversed range: 2001:db8::10-2001:db8::2
• invalid IPv6 endpoints
• compressed and expanded forms of the same IPv6 address

The fix changes the ordering semantics from lexical comparison to parsed IPv6 comparison, so these cases should verify both acceptance and rejection behavior.