fix(js): resolve relative URLs against the document base URL

`<base href>` overrides the document URL. Four call sites still
resolved against the document URL: `_anchorBase()`, the form's
`action`, `_resolveUrl()`, and the `src` reflection. An application
in a subpath therefore requested its chunks under the running route
and got 404.

`document_base_url()` in `crates/obscura-js/src/ops.rs` has resolved
per specification since #242, but it sat behind the `render` feature
and was not reachable from the JS layer. This commit enables the
function and passes its result through the `_domParse` dispatcher.

The resolution sits in a cache. Without it, reading `a.href` costs
many times more, because `document_base_url()` runs the selector
engine over all elements.