Support multiple filter overlays on Bar (Categorical) widgets

### Problem Statement

Dashboard widgets using the "Bar (Categorical)" display type only support a single filter condition. For time-series chart types (Line, Area, Bar Time Series), users can add up to 3 filter conditions via "+ Add Filter" to overlay multiple series on the same chart. This capability does not exist for categorical bar charts.

This means users who want to compare two related metrics side by side on a categorical axis (e.g., comparing counts of two different event types grouped by the same category field) must create separate widgets. This reduces at-a-glance visibility and doubles the number of widgets on a dashboard.

### Solution Brainstorm

Extend the "+ Add Filter" overlay support (currently available for time-series display types) to the "Bar (Categorical)" display type. This would allow multiple filter conditions per widget, with each filter rendered as a grouped or stacked bar series on the same categorical axis. The rendering layer would need to read beyond queries[0] to plot additional series.

### Product Area

Dashboards

Start by locating the existing "+ Add Filter" support for Line, Area, and Bar Time Series widgets, then compare how the rendering layer handles queries beyond queries[0]. Extend that behavior to Bar (Categorical) widgets, with each filter rendered as a grouped or stacked series on the same categorical axis. Done means multiple filter conditions can be added and displayed in one categorical bar widget.

Written by the indexing model from the issue text.