test: pin the embedded map and map merge rules

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -202,7 +202,8 @@ table with `Number`, which keeps the literal; see
 
 - `*struct`, matched per the field rules below
 - `*map[string]any` or `*map[string]T`, keys become map keys and values decode
-  into `T` recursively
+  into `T` recursively; a map that already holds entries is merged into, the
+  document's values replacing same-named keys and the rest left standing
 - `*OrderedMap`, the keys fill in the order the document wrote them; see
   [Ordered tables](#ordered-tables)
 - `*any`, receives the whole parsed tree unchanged
@@ -220,7 +221,9 @@ For a struct destination, a TOML key matches a field as follows:
    into the embedded struct and matches its own fields against the same keys,
    mirroring how the encoder flattens it. A nil embedded pointer struct is
    allocated on demand. An untagged embedded map receives the keys no field
-   claims.
+   claims; when a struct embeds several untagged maps, the first one
+   declared takes all of them and the rest stay untouched, so the rule stays
+   predictable.
 4. The key itself is lower-cased before lookup, so the match is
    case-insensitive on both sides: `DATABASEURL` matches a field named
    `DatabaseUrl`.