Add knob ablation test for visual regression rendering

[agentydragon]

Summary

Adds a comprehensive knob ablation test that systematically disables each rendering knob individually and compares the resulting screenshots against a baseline (all knobs enabled). This helps identify which knobs actually affect visual rendering and validates the hermetic rendering setup.

Key Changes

• New test file (tests/knob_ablation.mjs):

  • Tests 40+ rendering knobs across Chrome flags, CSS overrides, environment variables, media features, and viewport settings
  • Runs on two representative scenarios: DefinitionDetail (text-heavy) and DistributionChartRecall (chart rendering)
  • Uses pixelmatch with stricter threshold (0.1) to detect subtle rendering differences
  • Generates pixel-by-pixel diff images and JSON/Markdown reports
  • Includes timeout handling and process cleanup for flaky Chrome flag combinations

• Test runner script (tests/run_knob_ablation.sh):

  • Orchestrates local and remote (RBE) test execution
  • Collects artifacts from Bazel test outputs
  • Supports local, remote, or both modes

• Bazel integration (BUILD.bazel):

  • Adds knob_ablation test target with eternal timeout
  • Configures dependencies (Puppeteer, pngjs, pixelmatch, fonts, harness)
  • Sets up environment variables for hermetic rendering (FONTCONFIG_FILE, FREETYPE_PROPERTIES)

• Sample results (knob_ablation_results/):

  • Local test execution results showing which knobs affect rendering
  • Key findings: --disable-lcd-text and --font-render-hinting have measurable impact; most other flags are neutral

Implementation Details

• Knobs are categorized by type (chrome_flag, css, env, media, viewport) for organized testing
• Infrastructure flags (--no-sandbox, --single-process, etc.) are excluded from ablation
• Each knob test runs in isolation with its own browser process to prevent state leakage
• Handles Chrome crashes/hangs gracefully with per-knob timeouts and SIGKILL cleanup
• Generates both human-readable (Markdown table) and machine-readable (JSON) reports
• Supports environment variable override for execution context tracking

https://claude.ai/code/session_01FksebaqNvhq1t48eQ2AHGF