refactor(charts): extract shared chart-tooltip className to one source (#2106)

Closes #2011.

time-series and sankey each created their cursor-following tooltip with a
duplicated className literal that had already drifted apart: time-series was
missing `text-primary` and `z-50`. Move the visual contract into
app/javascript/utils/chart_tooltip.js as CHART_TOOLTIP_CLASSES and have both
controllers reference it. Each keeps its own behavioural classes (time-series
its initial `opacity-0`; both `top-0`; sankey toggles opacity via inline
style). `privacy-sensitive` stays bundled so future copies can't drop it.

Also exports a createChartTooltip factory for the raw-DOM idiom.

goal_projection_chart_controller is not in main yet (it lands with the goals
work in #1798); it migrates to the same symbol there.

app/javascript/controllers/sankey_chart_controller.js

@@ -1,6 +1,7 @@
 import { Controller } from "@hotwired/stimulus";
 import * as d3 from "d3";
 import { sankey } from "d3-sankey";
+import { CHART_TOOLTIP_CLASSES } from "utils/chart_tooltip";
 import { sankeyNodeHasChildren, zoomSankeyData } from "utils/sankey_zoom";
 
 // Connects to data-controller="sankey-chart"
@@ -509,10 +510,9 @@ export default class extends Controller {
     this.tooltip = d3
       .select(dialog || document.body)
       .append("div")
-      .attr(
-        "class",
-        "bg-container text-primary text-sm font-sans p-2 border border-secondary rounded-lg pointer-events-none absolute z-50 top-0 privacy-sensitive",
-      )
+      // Shared visual contract + this chart's positioning class; opacity is
+      // toggled via inline style below.
+      .attr("class", `${CHART_TOOLTIP_CLASSES} top-0`)
       .style("opacity", 0)
       .style("pointer-events", "none");
   }

app/javascript/controllers/time_series_chart_controller.js

@@ -1,5 +1,6 @@
 import { Controller } from "@hotwired/stimulus";
 import * as d3 from "d3";
+import { CHART_TOOLTIP_CLASSES } from "utils/chart_tooltip";
 
 const parseLocalDate = d3.timeParse("%Y-%m-%d");
 
@@ -287,10 +288,8 @@ export default class extends Controller {
     this._d3Tooltip = d3
       .select(`#${this.element.id}`)
       .append("div")
-      .attr(
-        "class",
-        "bg-container text-sm font-sans absolute p-2 border border-secondary rounded-lg pointer-events-none opacity-0 top-0 privacy-sensitive",
-      );
+      // Shared visual contract + this chart's initial-hidden / positioning classes.
+      .attr("class", `${CHART_TOOLTIP_CLASSES} opacity-0 top-0`);
   }
 
   _trackMouseForShowingTooltip() {

app/javascript/utils/chart_tooltip.js

@@ -0,0 +1,25 @@
+// Single source of truth for the cursor-following tooltip used by the chart
+// controllers (time-series, sankey, and goal-projection once it lands from the
+// goals work). Keeping the visual contract here stops the bg / text / border /
+// privacy-sensitive classes from drifting apart across the controllers, the way
+// they had before (time-series was missing `text-primary` and `z-50`).
+//
+// This is the VISUAL contract only. Callers append their own behavioural
+// classes (initial `opacity-0`, `top-0`, …) or set them via inline styles,
+// because how each chart shows/hides and positions its tooltip differs.
+//
+// Not to be confused with DS::Tooltip — that is the info-icon hint primitive
+// (bg-inverse, aria-describedby, anchored to a static trigger). This is a
+// data-card surface created and updated inside D3 handler code.
+export const CHART_TOOLTIP_CLASSES =
+  "bg-container text-primary text-sm font-sans absolute p-2 border border-secondary rounded-lg pointer-events-none z-50 privacy-sensitive";
+
+// Convenience factory for the raw-DOM idiom (no d3.select). Creates a hidden
+// tooltip div carrying the shared contract and appends it to `parent`.
+export function createChartTooltip(parent) {
+  const tooltip = document.createElement("div");
+  tooltip.className = CHART_TOOLTIP_CLASSES;
+  tooltip.style.display = "none";
+  parent.appendChild(tooltip);
+  return tooltip;
+}