feat: add the required tag option and UnmarshalerContext

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -46,6 +46,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   (10000 levels, which no hand-written document approaches): a document that
   nests arrays or inline tables deeper used to run the stack out and is now
   rejected with a `SyntaxError` naming the limit.
+- 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
+  resolves to it. The option shapes decoding only, and the encoder ignores
+  it.
+- `UnmarshalerContext`, the custom-decode interface that hands the decode's
+  context to the method, `UnmarshalTOMLContext(ctx, data)`. It wins over
+  `UnmarshalTOML` when a type implements both, so a long custom decode can
+  abort on cancellation; a non-cancellable entry point hands in
+  `context.Background`, never nil.
 - A TOML array decodes into a Go fixed-size array, `[N]T`, where only a slice
   was accepted before; the encoder could already encode one. A length mismatch
   is an error wrapped with the key path.