fix(dom): bridge <template> contents to JS so .content is not always empty (#463)

The parser already put template children in a separate contents document
per the HTML spec (tree_sink.rs), but nothing exposed that node, so
`get content()` fabricated an empty DocumentFragment and the parsed
markup was unreachable:

    document.body.innerHTML = '<template id=t><p>a</p></template>';
    t.content.childNodes.length   // 0 (Chrome: 1)
    t.innerHTML                   // "" (Chrome: '<p>a</p>')

Every <template>-driven UI — custom elements cloning
`template.content`, lit, Vue runtime templates, htmx fragments, Alpine
x-if/x-for — stamped nothing, with no error raised.

Three layers were involved:

- `DomTree::template_contents` returns the contents node, allocating one
  on demand so templates built via `createElement` work too. Exposed as
  the `template_contents` op; `get content()` wraps it as a
  DocumentFragment cached by node id, so `.content` keeps a stable
  identity across reads.

- `import_node_from` cloned the element data verbatim, carrying the
  *source* tree's contents NodeId into the destination tree, where it
  indexed whatever unrelated node occupied that slot. That is why a
  template injected via innerHTML resolved `.content` to a bogus node
  rather than merely an empty one. The contents are now imported into a
  fresh node and the reference remapped, queued onto the existing
  worklist so nested templates stay iterative.

- The serializer emitted a template's own (always empty) children, so
  contents vanished from outerHTML/innerHTML and `cloneNode(true)` —
  which round-trips through outer_html — produced an empty template.
  `content_source` resolves a template to its contents document for both
  inner_html and outer_html.

With serialization fixed on the Rust side, the `set innerHTML` template
branch that routed to `.content` now has a matching getter for free, so
no template special case is needed on the JS hot path.

crates/obscura-js/js/bootstrap.js

@@ -1216,6 +1216,8 @@ class Element extends Node {
     if (this.localName === "svg") return "http://www.w3.org/2000/svg";
     return "http://www.w3.org/1999/xhtml";
   }
+  // `inner_html` resolves a <template> to its contents document on the Rust
+  // side (issue #463), so this needs no template special case.
   get innerHTML() { return _domParse("inner_html", this._nid) ?? ""; }
   set innerHTML(v) {
     if (this.localName === 'template') {
@@ -1254,6 +1256,18 @@ class Element extends Node {
     // infinite retry loop (issue #210).
     const tag = this.localName;
     if (tag === 'template') {
+      // Back the fragment with the node's real template contents (issue #463).
+      // The parser stores template children in a separate contents document
+      // instead of under the element, so without this the getter handed back a
+      // fabricated empty fragment and the parsed markup was unreachable.
+      // `template_contents` allocates one on demand for created templates.
+      const nid = +_dom("template_contents", this._nid);
+      if (nid >= 0) {
+        // Cache by node id so `.content` keeps a stable identity across reads —
+        // frameworks stash the fragment and compare it later.
+        if (!_cache.has(nid)) _cache.set(nid, new DocumentFragment(nid));
+        return _cache.get(nid);
+      }
       if (!this._templateContent) this._templateContent = document.createDocumentFragment();
       return this._templateContent;
     }