Boost page navigation with htmx

Links and GET forms to wiki pages swap the next page into .shell instead
of loading a new document: the server answers boosted requests and
history restores with a partial (title, page actions out-of-band, shell
content). The header stays, browsers without cross-document view
transitions (Firefox) no longer flash between pages, and the swap runs
as a same-document view transition. Files, the editor, internal
endpoints and POST forms still navigate normally.

Back/Forward refetch pages (no snapshots that could predate a move or
delete); the tree rail keeps its scroll position and the content column
returns to where it was. Mutation redirects swap their result in the
same way. Page scripts that assumed a fresh document now also run after
a swap (toc, contents, companion, scroll-sync, search actions) or
register once (fitness).

The e2e suite now also runs in Firefox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assets/style.css

@@ -425,6 +425,8 @@ main > h2 {
 .dropdown-menu.scrollable { max-height: 23rem; overflow-y: auto; }
 
 .actions-special > .btn {text-align: left;}
+/* Wrapper for the out-of-band swapped page actions; lays out as if absent. */
+.page-actions { display: contents; }
 /* Header actions carry an icon and a label: the mobile dropdown shows the
    label, the inline desktop bar (see responsive block) the icon. */
 .btn-icon { display: none; }