feat: bound the encoder walk and add UnmarshalWithOptions

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -52,6 +52,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   order on decode and sorts on encode. It works as a decode target on its
   own, in a struct field, and as the element of an array of tables; its
   values are untyped, so a nested table stays a `map[string]any`.
+- `UnmarshalWithOptions(data, v, opts)` decodes with a `DecodeOptions` struct
+  in one call, the options a `Decoder` sets without building one: unknown
+  keys, `Number` literals, and the parse limits.
+- `Marshal` carries a nesting limit of 10000 levels, the parser's own figure:
+  cyclic data, which used to run the stack out, is now rejected with an error
+  that names the limit and the path it was met at.
 - The `toml` tag gained the `required` option: a field tagged
   `toml:"host,required"` makes the decode fail with
   `missing required key "host"` when the document carries no key that