create_parity_edge

Create the parity / rivalry edge `feature_rivals_competitor_feature` from our `feature` to a `competitor_feature`, carrying the assessment (parity_status / quality / is_gap / assessed_on / evidence / confidence) as edge metadata. A typed convenience over the generic edge writers: it fixes the edge type, validates the parity enums, derives `is_gap` from `parity_status` when omitted, and routes automatically. Within the active graph it writes a catalogue edge (like `create_edge`); cross-product (their `competitor_feature` in a separate watched intelligence graph) it writes a cross-edge (like `create_cross_product_edge`), with the our-side product defaulting to the active product. The edge is authoritative; the node `parity_status` is a denormalised single-rival cache that `validate_graph` checks for divergence.