Fix staged binary assets in browser previews

[chubes4]

Summary

• keep the programmatic Playground preview on one PHP instance so setup/materialization writes and browser HTTP requests observe the same VFS
• add a browser-level integration that stages a real PNG and HTML under WordPress uploads
• verify intrinsic dimensions, visible geometry, and exact canvas-painted RGBA bytes

Root cause

materializePlaygroundStagedInputs() writes staged binary bytes through the primary PHP instance. The programmatic request handler previously allowed secondary PHP instances for browser HTTP requests; those instances did not receive post-boot VFS writes, so staged HTML/CSS could load while PNG/JPEG assets failed.

Setting maxPhpInstances: 1 makes runtime setup, staged input materialization, and browser preview requests share one coherent filesystem.

Closes #1935.

Verification

• npm run build
• npm run test:playground-staged-upload-preview
• npx tsx tests/staged-input-materialization.test.ts
• npm run test:browser-visual-compare-capture-reliability
• npm run test:playground-readonly-mounts-integration
• Linux Lab: npm run test:playground-staged-upload-preview

Focused Fisiostetic Lab rerun with this branch:

• source images: 26/26 loaded, 0 failed (previously 15 loaded, 11 failed)
• candidate images: 17/17 loaded, 0 failed
• source/candidate capture readiness: high
• mismatch: 11.90% (837041/7036160), improved from 19.58% (1377921/7036160) without Blocks Engine or SSI changes

The remaining mismatch is real source/candidate geometry rather than missing source assets.

AI assistance

• AI assistance: Yes
• Tool: OpenAI GPT-5.6 Sol via OpenCode
• Used for: Root-cause analysis, implementation, integration coverage, and Lab verification.