trends: staggered per-series line draw-in

The single-waterline trace-in was invisible with sparse data (2 points draw
in one step). Replace it with a per-series waterline: each station's line now
traces left->right on first appearance, series revealed 0.18s apart in a
cascade so the draw is clearly visible. Newly-added favourites get a full
reveal immediately on reload; Reduce Motion still jump-cuts. Range/mode
morphs and numeric headline transition unchanged.

FuelBoard/TrendsView.swift

@@ -35,10 +35,12 @@ struct TrendsView: View {
     @State private var loadFailed = false
     @State private var firstSnapshot: String?
 
-    /// Lines trace-in left→right on first appearance by revealing an
-    /// increasing prefix of each series' points. Stays at the full count after
-    /// the first reveal so range/mode switches morph instead of re-tracing.
-    @State private var revealCount: Int = 0
+    /// Per-series draw-in waterline: maps stationID → how many leading points
+    /// are revealed, so each line traces left→right on first appearance.
+    /// Series reveal in a short staggered cascade so the draw is clearly
+    /// visible even with only 2 points per line. Stays full after the first
+    /// reveal so range/mode switches morph instead of re-tracing.
+    @State private var revealed: [String: Int] = [:]
 
     /// Seeded from `selectedFuel` (the fuel the tab was on) so the sheet
     /// opens where the user was — same pattern as FavouritesView.
@@ -171,16 +173,32 @@ struct TrendsView: View {
             withAnimation(reduceMotion ? nil : .easeInOut(duration: 0.35)) {
                 series = fetched
             }
+            // Newly-added favourites have no reveal entry yet — surface them
+            // fully so their line isn't left blank by the draw-in waterline.
+            for h in fetched where revealed[h.stationID] == nil {
+                revealed[h.stationID] = h.points.count
+            }
         } else {
             series = fetched
-            let maxPoints = fetched.reduce(0) { max($0, $1.points.count) }
-            if maxPoints > 0 {
+            let hasPoints = fetched.contains { !$0.points.isEmpty }
+            if hasPoints {
                 if reduceMotion {
-                    revealCount = maxPoints
+                    var all: [String: Int] = [:]
+                    for h in fetched { all[h.stationID] = h.points.count }
+                    revealed = all
                 } else {
-                    revealCount = 0
-                    withAnimation(.easeOut(duration: 0.5)) {
-                        revealCount = maxPoints
+                    revealed = [:]
+                    let cascadeNS = UInt64(0.18 * 1_000_000_000)
+                    for (i, h) in fetched.enumerated() {
+                        let sid = h.stationID
+                        let total = h.points.count
+                        Task {
+                            try? await Task.sleep(nanoseconds: UInt64(i) * cascadeNS)
+                            guard !Task.isCancelled else { return }
+                            withAnimation(.easeOut(duration: 0.5)) {
+                                revealed[sid] = total
+                            }
+                        }
                     }
                 }
             }
@@ -419,13 +437,13 @@ struct TrendsView: View {
         // type-checker's budget.)
         Chart {
             ForEach(displaySeries) { history in
-                ForEach(history.points.prefix(revealCount)) { point in
+                ForEach(history.points.prefix(revealed[history.stationID] ?? 0)) { point in
                     areaMark(point, series: history.name,
                              color: seriesColor(index(of: history.stationID)))
                 }
             }
             ForEach(displaySeries) { history in
-                ForEach(history.points.prefix(revealCount)) { point in
+                ForEach(history.points.prefix(revealed[history.stationID] ?? 0)) { point in
                     lineMark(point, series: history.name,
                              color: seriesColor(index(of: history.stationID)))
                 }