v3.1.0-rc6

Release candidate 6 for v3.1.0.

Review-round fixes on top of rc5:

• Atmo: legacy slug-style locations (e.g. location: nice from pre-discovery configs) now map to the correct discovered instance. resolveEntityIds() tries the slug-to-config_entry_id mapping before falling back to the old buildEntityId path, so prefixed multi-instance setups work for legacy configs too.
• Card: header title for Atmo resolves legacy slugs via discovery instead of silently defaulting to the first discovered location's label.
• Editor: location dropdown keeps the legacy slug visible as a compat entry when it matches a discovered instance, so existing configs stay editable.
• Test: corrected an inaccurate comment about GP display_name classification.

Nothing else changed since rc5.