fix(encode): emit one key when two fields resolve to one name

Assisted-by: GLM 5.3

CHANGELOG.md

@@ -49,6 +49,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   schema walk recursed through the embedded type forever, so such a
   `Unmarshal` call hung the process; the walk now tracks the struct types on
   the current path and stops when one repeats.
+- `Marshal` emits exactly one key when two struct fields resolve to the same
+  TOML name, picking the field the decoder would fill (the shallower one, the
+  later declaration at equal depth). Such a struct previously marshalled into
+  a duplicate key, and the output never re-parsed, breaking the round-trip
+  guarantee.
 - An array-of-tables header whose path runs through an inline table
   (`a = {b = {}}` followed by `[[a.b.c]]`) is rejected. The frozen-inline-table
   check covered `[table]` headers and dotted keys but not the intermediate