fix: report single-member required xor groups as missing

[Solaris-star]

Summary

When a required flag is in an xor group with optional alternatives, an empty parse previously reported no missing flags if the group only contributed a single required summary (len(flags) > 1 gate).

That made cases like:

Hello string `xor:"hello" required:""`
One   string `xor:"hello" and:"two"`
Two   string `and:"two"`

accept Parse([]string{}) without error (#516).

Fix

In checkMissingFlags, report required xor groups when no member is set even if only one required summary is present (len(flags) >= 1).

Test plan

• go test .
• New TestXorAndRequiredIssue516
• Existing xor/and required tests still pass

Fixes #516