feat: decode arrays into [N]T and add MarshalAppend

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -158,6 +158,11 @@ out, err := interpres.Marshal(cfg)
 The cancellable variant of `Marshal`. The context is checked before any work
 and every 64 fields during the reflection walk.
 
+### `func MarshalAppend(buf []byte, v any) ([]byte, error)`
+
+Appends the TOML encoding of `v` to `buf` and returns the extended buffer, the
+shape `json.MarshalAppend` has. A failed encoding leaves `buf` untouched.
+
 ## Decoding
 
 ### Value mapping
@@ -282,6 +287,11 @@ destination is a slice, each element decodes into the slice's element type
 (`[]struct` or `[]map[string]V`); a mismatch on one element surfaces as an
 error wrapped with `[i]:` and the element index.
 
+A value array also decodes into a fixed-size array, `[N]T`, the mirror of the
+encoder's ability to encode one. The element count has to match: an array
+whose length differs from `N` is an error, `interpres: cannot assign 2
+elements to [3]int`, wrapped with the key path.
+
 ### Custom decoding: `Unmarshaler`
 
 A type that wants full control of its decode implements: