[T-20260709-0005] Document visual testing workflow and baseline management

[georgi]

Committed. Here's the summary.

Summary

Created VISUAL_TESTING.md and linked it from the main README, giving the team a clear process for managing visual baselines and distinguishing intentional changes from regressions.

What changed

• VISUAL_TESTING.md (new, repo root) — documents the hybrid visual regression stack from the accepted plan: Storybook + Chromatic for component/design-system coverage and Playwright toHaveScreenshot() for critical E2E flows (node graph, settings, chat). Sections: How Visual Tests Work (component vs E2E, Chromatic workflow, Playwright workflow, baselines & diffs), Accepting Changes (Chromatic UI steps + npm run test:visual:update baseline commit flow), Common Scenarios, Troubleshooting (font rendering, animation timing, viewport mismatch, theme toggle, missing baselines, WebGPU/Sketch in CI), Team Workflow, Configuration Files (.chromatic.config.json, playwright.config.ts visual settings, .github/workflows/visual-tests.yml), Determinism, and a Quick Reference table.
• README.md — added a one-line link in the Testing section pointing to VISUAL_TESTING.md.

Key decisions / caveats

• The doc is grounded in the actual repo harness: it references the real-backend web/tests/globalSetup.ts, the waitForAnimation/waitForPageReady determinism helpers, the existing playwright.config.ts settings (1920×1080 viewport, retries:0, workers:1, SwiftShader WebGPU args), and explicitly disambiguates the capture-only npm run screenshots doc-screenshot workflow from the regression layer.
• Storybook, Chromatic config, Playwright visual assertions, and visual-tests.yml are owned by the sibling in-progress tasks (T-20260709-0002/0003/0004). I documented their intended commands/paths (matching the task's npm run test:visual:update / tests/__snapshots__ contract) without creating those files, to avoid merge conflicts with the parallel branches. The doc describes the target steady-state plus the plan's non-blocking enforcement posture.
• Acceptance criteria all satisfied: VISUAL_TESTING.md created, team workflow documented, accepting-change examples included, troubleshooting section present, linked from README.

Verification

• prettier --check VISUAL_TESTING.md → passes (formatted with the repo's .prettierrc).
• Confirmed all internal anchor links map to real headings and relative file links resolve; the repo's lychee link check is advisory (fail: false) and was not installed locally.
• README was already prettier-dirty on origin/main before this change; I left the rest untouched and only added the single linking line to avoid unrelated churn. No typecheck/lint applies to a Markdown-only change.

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Closes task T-20260709-0005: Document visual testing workflow and baseline management.

Acceptance criteria

• VISUAL_TESTING.md created with clear instructions
• Team workflow documented
• Examples for accepting changes
• Troubleshooting section with common issues
• Linked from main README