WholeRecordBuilder

Everything else — enums, bytes, logical types, collections, sums, value classes — encoded by the field type's own vulcan codec, summoned at the derivation site and held here.

Primitive leaf written directly: the runtime value IS the Avro datum (Boolean / Int / Long / Float / Double / String). Validated against the schema field type at construction.

One case field: its name and how toRecord obtains the datum it puts.

Option[X]: None puts null (vulcan's own OptionCodec), Some(v) puts through the inner arm against the non-null union branch.

Nested case class: a sub-record level built by the same rule against the field's own record schema — the recursion that makes nested shapes cost leaf-by-leaf, not codec-per-sub-record.

One record level: typeName for failure messages, case fields in declaration order.

A self-recursive ancestor already on the derivation path (depth levels up, 0 = the level being built): resolved to that level at construction, so recursive case classes terminate.

Assemble a WholeRecordBuilder for A from schema (the codec's) and the compile-time shape. Returns the failure — naming the field and the record — instead of a builder, when a case field names no schema column, two case fields claim one, or an arm disagrees with its schema field's shape. Never throws.