ACF row naming: pick flexible layout from acf_fc_layout; key+name collision

[ABB65]

Follow-ups from #44 review (t2), non-blocking:

1. A sub-field name shared by two flexible layouts resolves to the last layout's definition in Source::acf_value; choose the layout by the row's acf_fc_layout.
2. A row holding both a sub-field's key and its name: the later one overwrites silently; report it (e.g. acf-key-name-collision).