feat(visualizer): absorb SimpleAudit result visualizer into Studio

[SushantGautam]

Ports the single-file HTML result viewer (formerly simpleaudit serve / export-html in the core) into Studio as Django views plus two self-contained static assets.

What's here

• infra/visualizer.py — file-tree, JSON/image APIs, standalone HTML export, and the /visualizer/ + /visualizer/upload/ SPA pages
• config/urls.py — routes for the SPA and its APIs. The SPA hardcodes /api/* fetch paths prefixed by __VISUALIZER_API_BASE, so the API routes live under /api/visualizer/api/* to match
• simpleaudit_studio/cli.py — --results_dir + --visualize-only to browse a folder of dumped results server-side
• templates — export-html entry points on run detail/result and compare
• tests — shape detection, file tree, JSON/image endpoints, path traversal, login gating, standalone export

Verification

• Full infra suite: 435 passed, 1 skipped (needs DEMO_MODE)
• ruff clean
• Browser end-to-end: file tree, single-file view, and experiment model picker all render with zero JS/console errors

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