feat: bound the encoder walk and add UnmarshalWithOptions

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -73,6 +73,13 @@ if err := interpres.Valid(data); err != nil {
 }
 ```
 
+### `func UnmarshalWithOptions(data []byte, v any, opts DecodeOptions) error`
+
+The one-shot form of a configured `Decoder`: the same options as
+`NewDecoder` sets, in a `DecodeOptions` struct, applied to a single call.
+The zero value takes the defaults: unknown keys ignored, numbers evaluated
+as `int64` and `float64`, no size limit and the 10000-level nesting default.
+
 ## Documents
 
 `Parse` returns a `Document`: the value tree together with what a map cannot
@@ -443,6 +450,11 @@ different top-level value fails:
 | a nil `any` | `interpres: cannot marshal nil value` |
 | a nil pointer | `interpres: cannot marshal nil pointer` |
 
+The encoding walk carries a nesting limit of 10000 levels, the parser's own
+figure: a value that nests deeper, which cyclic data always does, is rejected
+with an error that names the limit and suggests the cycle, instead of running
+the stack out.
+
 ### Field matching
 
 Struct fields become TOML keys as follows:
@@ -739,9 +751,11 @@ same way.
 ### `type Decoder`
 
 Configurable strictness for decoding, constructed with `NewDecoder`. Set up
-with `DisallowUnknownFields`, then call `Decode` or `DecodeContext` any number
+with the chainable methods, then call `Decode` or `DecodeContext` any number
 of times. A configured `Decoder` holds no per-call state and is safe for
-concurrent use.
+concurrent use. For a single document, `UnmarshalWithOptions(data, v,
+DecodeOptions{...})` sets the same options without the Decoder; its zero
+value takes the defaults.
 
 | Method | Default | Effect |
 |---|---|---|