fix: support opacity and shape in observable plot renderer

[ObservedObserver]

Motivation

• The Observable Plot renderer did not map Vega-Lite shape and opacity encodings, so plotted marks could miss symbol and transparency information.
• Both encoded opacity (enc.opacity.field) and constant opacity (enc.opacity.value) should be supported for accurate rendering.
• Point-like marks need to map Vega-Lite shape to Plot's symbol channel to reflect different marker shapes.

Description

• Extract shapeField, opacityField, and opacityValue from the Vega-Lite encoding and add them to the fields passed into createMark.
• Add opacity handling in createBaseConfig so baseConfig.opacity is set from either a field or a constant value (enc.opacity.field or enc.opacity.value).
• Map shapeField to the Plot symbol channel for point, circle, and dot marks by setting baseConfig.symbol.
• Added logging of the new fields and updated the fields object to include shapeField, opacityField, and opacityValue.

Testing

• No automated tests were run against this change.
• Existing unit tests were not modified by this PR.

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Codex Task

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Note

Adds missing channel mappings to better reflect Vega-Lite specs in Observable Plot.

• Extracts shapeField, opacityField, and opacityValue from encoding and threads them through fields
• Sets baseConfig.opacity from either enc.opacity.field or enc.opacity.value
• Maps shapeField to Plot symbol for point, circle, and dot marks
• Updates debug logging to include new fields

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Project	Deployment	Review	Updated (UTC)
graphic-walker	Ready	Preview, Comment	Jan 1, 2026 9:06am