Function collect_json2_rewrite_plans 

pub(crate) fn collect_json2_rewrite_plans(
    metadata: &RegionMetadataRef,
    schemas: &[(SchemaRef, u64)],
) -> Result<HashMap<String, Json2RewritePlan>>

Builds JSON2 rewrite plans from existing physical schemas.

Each source row count weights all of its existing explicit leaves. Ordinary paths are never discovered from the v2 remainder, so this operation cannot unexpectedly promote opaque data.