[nishantmonu51]

Dimension comparison on the explore page renders the shared Vega stacked bar, and nested Druid columns surface as dimensions whose names contain dots (for example query.context.metaData.type). The hover layer's pivot transform received that name unescaped, so Vega read it as a nested field path, threw inside the dataflow, and every time series chart rendered blank.

• Escape the color dimension in createVegaTransformPivotConfig (used by the stacked, normalized, grouped bar and line charts), in the area chart's inline pivot, and in the grouped bar xOffset encoding.
• Sweep the remaining spec builders for raw names handed to Vega as field references and escape them: the hover layer x field (which also restores custom x sorts for dotted dimensions), the normalized bar joinaggregate, the funnel window, fold, color and label encodings, the multi-measure, comparison and combo fold inputs, and the heatmap and pie label text. fold.as keeps the raw name because Vega emits the raw name as the fold key.
• Add spec tests for every escaped slot via a vegaFieldReferences helper that collects each field reference in a spec, plus a headless Vega render test that fails on the unescaped name.

Steps to reproduce:

1. Open an explore whose metrics view has a dimension with a dot in its name (any Druid nested column).
2. Turn on dimension comparison for that dimension and select two values in the leaderboard.
3. Before this change the measure charts are blank; after it the stacked bars render.

Closes APP-957

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!