Gauge / progress-to-target insight display type (actuals vs plan)

Is your feature request related to a problem?

People tracking actuals vs plan (revenue vs quota, signups vs goal) have no way to see "% of target" as a single visual in insights. Goal lines only overlay time-series charts, the number (BoldNumber) view has no target concept, and pie charts show share of a whole rather than a value against a target. This request comes up regularly.

Describe the solution you'd like

A new insight display type (for example Progress, or a gauge) for single-value trends and SQL number insights. It renders the aggregate against a target as a filled bar or radial ring with a "% of target" label.

A design suggestion to keep it cheap: reuse the existing goal-line value as the target. Read the first GoalLine.value off TrendsFilter.goalLines instead of adding a new field, so there's no new schema or editor, and the goal-line label doubles as the gauge label.

Scope looks frontend-only:

• Trends already return the single aggregated_value that the number view reads, so no backend query change.
• New renderer alongside BoldNumber (frontend/src/scenes/insights/views/BoldNumber/).
• One ChartDisplayType enum value (frontend/src/types.ts), a picker entry in ChartFilter.tsx, a dispatch branch in Trends.tsx, and extending the goal-line display gate to the new type.
• Ship behind a feature flag.

A bar or ring version is small. A full semicircular dial is larger, because no charting library currently in use (Chart.js, quill-charts) supports partial arcs out of the box.

Describe alternatives you've considered

Workarounds available today:

• Trend chart with a goal line: a dashed target line on a line/bar/area chart. Closest option now, but it needs a time series and can't render a single "% of target" figure.
• HogQL number insight: a SQL insight computing round(100.0 * actual / target, 1) shown with the Number display. Gives the figure, but no visual fill and no % sign.

Neither gives a glanceable gauge or progress visual.

Additional context

Reusable building blocks already exist but aren't wired into insights:

• LemonProgress (linear bar) and LemonProgressCircle (radial ring, already used in experiments RunningTime).
• quill PieChart donut mode with centerLabel (see products/mcp_analytics HarnessDonut).
• The billing BillingGauge bar.

Debug info

- [x] PostHog Cloud (US) — product enhancement request, not version-specific
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit:
- [ ] PostHog self-hosted with Kubernetes

Start with frontend/src/types.ts, ChartFilter.tsx, Trends.tsx, and the existing renderer under frontend/src/scenes/insights/views/BoldNumber/. Review LemonProgress and LemonProgressCircle, then trace how TrendsFilter.goalLines and aggregated_value reach the number view. Done means a feature-flagged display type renders actuals against the first goal-line value and is selectable in the picker without backend changes.