panel-redesign: ratchet list, fixture registry and a route seam for the guards

Groundwork the guard tests of stage 0 stand on; no behaviour changes.

  - internal/web/view/legacy_pages.txt: the redesign ratchet. Every page of
    today's panel is listed as still old, plus @kit and @routes for the two
    pieces of work whose tests stay quiet until they land. The list only
    shrinks (enforced by CI in the next commits).
  - internal/web/view/fixtures_test.go: the registry of fixtures the guards
    render redesigned pages with. Empty until the kit page exists.
  - web.go: route registration moves out of Handler() into muxes(), so a
    test can ask the real muxes which pattern answers a path.

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

internal/web/view/legacy_pages.txt

@@ -0,0 +1,36 @@
+# The redesign ratchet (docs/plans/panel-redesign.md § Against resurrected pages).
+#
+# One entry per line: what is still pre-2.0 and therefore not yet held to the
+# design contract by the guard tests. This list ONLY SHRINKS — CI fails a
+# commit whose list is not a subset of its parent's (.github/scripts/design-first.sh).
+# Finishing a piece of work means deleting its line in the same commit; from
+# then on the guards hold that piece for good.
+#
+#   @kit     the component kit is not complete (stage 0): panel.css,
+#            components.html, components.go, layout.html, GET /server/components
+#   @routes  the pre-2.0 route table is still in place (stage 1)
+#   <name>   a page of the view engine that is still the old template
+@kit
+@routes
+backup
+dashboard
+deliveries
+delivery
+dmarc
+dmarc_domain
+dmarc_report
+domain_delete
+domain_detail
+help
+inbound
+inbound_delete
+inbound_domain
+login
+mail_queue
+settings
+setup
+status
+system_log
+user_delete
+user_form
+users