docs: note device-pixel vs CSS-pixel mismatch in screenshots

[sauravpanda]

Summary

• capture_screenshot() writes PNGs at device pixels but click_at_xy() takes CSS pixels — on a 2x display, reading a coordinate off the image and clicking it directly misses by half.
• Flesh out interaction-skills/screenshots.md with the conversion recipe and the discovery-vs-verification distinction, and add a docstring to click_at_xy pointing at it.

Verification

Confirmed the gap empirically before writing the doc:

screenshot pixels: (4592, 2286)
window: {"dpr":2,"iw":2296,"ih":1143}

Then dropped a 60×60 button at CSS (180,180) and clicked twice:

• click_at_xy(210, 210) → click registered (CSS coord inside button) ✅
• click_at_xy(420, 420) → no click (device-pixel coord, lands well outside) ❌

So click_at_xy definitively takes CSS pixels, and the previous screenshots.md (one sentence) didn't say so.

Test plan

• browser-harness -c "print(click_at_xy.__doc__)" shows the new docstring
• Helpers still import and page_info() still works
• Reviewer skim of interaction-skills/screenshots.md for tone/scope

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Summary by cubic

Clarify the device‑pixel vs CSS‑pixel mismatch between capture_screenshot() and click_at_xy(), and add a docstring that points to a short conversion recipe in interaction-skills/screenshots.md. This prevents off‑by‑DPR clicks on HiDPI/Retina displays and explains when to use full‑page vs viewport screenshots.

^{Written for commit dcd8137. Summary will update on new commits.}

[browser-harness-review]

✅ Skill review passed

Reviewed 1 file(s) — no findings.

[cubic-dev-ai]

No issues found across 2 files