#2911 gatorwalk-factory: remove the static design page (design_page and its Mermaid renderer); the studio is the only visualization

Remove the factory's static export: the design_page method, its design-page file resource, and the HTML and Mermaid renderer. The studio is the only visualization.

Why

Seth decided on 2026-10-01 to drop all static export functionality for now: "we have the studio. leave it at that for now." #2811 (a static studio export to replace this page) is closed for the same reason. Without this change, public users get two visualizations at launch: the Mermaid page from #2688 and the studio. Since #2839, the authoring skill's "Show it" step uses the studio, not this page.

Scope

• Remove the design_page method and the design-page resource spec from the factory model type (extensions/models/engine/factory.ts). Remove DESIGN_PAGE_SPEC, DESIGN_PAGE_NAME and the method body from _lib/engine/work_item_ops.ts.
• Keep designView and its DesignView types. The studio page bundles them (DESIGN.md, the studio section: the page runs DefinitionSchema, analyzeDefinition and designView). Remove renderDesignPage and everything only it uses: the HTML builders, Mermaid source generation, the inline CSS and script, and the Mermaid CDN and SRI constant. Move what remains to a module named for what it is, for example _lib/engine/design_view.ts, and update the studio's imports. Then rebuild the studio assets (deno task build:studio), so studio_assets_test stays green; #2886 shows what happens if you don't.
• Tests: remove the renderer's tests and keep or move the designView tests. Remove the design_page integration tests and any skill-command checks for it.
• Docs:
  • Remove design_page and design-page from the README (vocabulary, layout, commands) and from the skill (references/driving.md lists the method and its swamp data get command).
  • In DESIGN.md, mark "The design page" section as removed, keeping the parts about designView that the studio relies on.
  • Add a decision-log entry citing Seth's decision and #2811.
  • Grep for "design page" and "design_page" across gatorwalk-factory, including the rename script's fixtures, and leave nothing stale.
• No model version bump and no upgrade: the extension has never been published.

Done when

• swamp model method run <factory> design_page is an unknown method.
• No design-page data is written by any method.
• The studio's Design and Simulate modes still work: studio tests pass, studio_assets_test passes, and a manual load of the studio on an example shows the graph.
• No mention of design_page, design-page or Mermaid remains in gatorwalk-factory's code, skill or README.

Out of scope: any studio feature work. Other studio items are running: #2841 and #2889 (file-watch). Rebase onto origin/main before the PR.