Preserve inherited presence and validation

docs/config.md

@@ -121,6 +121,14 @@ sequence, so a step cannot extend another step. Replacing `steps` or a validator
 chain supplies the complete replacement sequence; entries are not appended or
 merged by identifier.
 
+Inherited values count as explicit input for Notarius's existing
+presence-sensitive rules: zero, false, and empty-list overrides stay explicit
+instead of restoring a default. Null also replaces the inherited value, but
+each field's ordinary type and validation rules still apply; null is not
+universally valid. Omitting `extends` preserves a relationship supplied by a
+lower-priority source, while `extends: null` cancels that relationship. It
+does not independently clear other inherited fields.
+
 `extends` is now a reserved key in every input mapping, including dynamic
 module options and named maps. Existing ordinary data or entry names using that
 key must be renamed along with any consumer that reads them. Configuration

docs/roadmap/implementation.md

@@ -168,6 +168,8 @@ syntax section. Presence-sensitive cases belong to Stage 3.
 
 ## Stage 3 — Preserve Presence And Existing Domain Validation
 
+> Completion: complete.
+
 Goal: ensure inherited input remains explicit through Notarius normalization,
 defaulting, and validator-chain conversion.