feat: add mark.trendline opt-out for scatter regression line

[rileyhilliard]

Problem

Scatter charts render a least-squares regression trend line unconditionally, with no way to turn it off. When a chart draws its own reference line — e.g. a manual x=y diagonal in a separate layer — the auto trend line adds a second, competing diagonal.

The AISD-takeover D/F scatter has an intentional x=y reference line; the auto trend line drew a misaligned dashed diagonal on top of it.

Fix

Add mark.trendline (default true, so existing charts are unchanged) and gate the renderer on markDef.trendline !== false. Authors suppress the auto line with mark: { type: 'point', trendline: false }.

Tests

• renders a trend line by default
• suppresses the trend line when mark.trendline is false

Engine suite: 1573 passing.

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 51cd77d and 6be87f2.

📒 Files selected for processing (4)

• CHANGELOG.md
• packages/core/src/types/spec.ts
• packages/engine/src/charts/scatter/__tests__/compute.test.ts
• packages/engine/src/charts/scatter/index.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/scatter-trendline-opt-out

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