test(editor): guard #146 contentDOM-first invariant + fix code-block comment (PR #147 review)

Addresses the #147 review (Approve with comments):
- Add footnote-views.structure.test.tsx: a structural regression guard asserting
  the editable NodeViewContent is the FIRST child of FootnotesListView and
  FootnoteDefinitionView, with no contenteditable=false chrome before it. The
  whole #146 fix rests on this DOM-order invariant; the macOS caret symptom needs
  a real browser, but the order proxy is testable in jsdom. Stubs @tiptap/react
  so the views render as plain DOM — the test passes on the fixed order and fails
  on the pre-fix chrome-first order.
- Reword the code-block-view comment: it claimed a "top-right overlay (the
  transclusion pattern)", but the menu stays fully in flow as a full-width row
  lifted via flex `order: -1` (the .codeBlock wrapper is a flex column). No
  overlay/absolute positioning.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/editor/components/code-block/code-block-view.tsx

@@ -50,8 +50,10 @@ export default function CodeBlockView(props: NodeViewProps) {
       {/* #146: the editable <pre><code> (contentDOM) MUST come first in the DOM.
           With the non-editable menu rendered before it, the browser's click
           hit-testing snapped the caret up one line. Render content first; the
-          menu follows and is positioned as a top-right overlay via CSS (the
-          transclusion pattern), so it no longer sits in-flow above the code. */}
+          menu is rendered after it and lifted back above visually via flex
+          `order: -1` (the `.codeBlock` wrapper is a flex column — see
+          code-block.module.css). It stays fully in flow as a full-width row
+          above the code: no overlay/absolute positioning. */}
       <pre
         spellCheck="false"
         hidden={