feat: animate fleet of container ships along bay waypoints

Replace the static ship marker with three signal-backed ships that
lerp along waypoint loops through SF Bay (Golden Gate, Oakland, and
Sausalito ferry routes). All clients see the same ship positions via
shared Go state synced every second.

internal/examples/maplibre/main.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"fmt"
+	"math"
 	"math/rand"
 	"strconv"
 	"sync"
@@ -24,6 +26,55 @@ var (
 	}
 )
 
+// shipState tracks a ship lerping along a loop of waypoints.
+type shipState struct {
+	lng, lat float64
+	waypoints [][2]float64 // [lng, lat] pairs
+	wpIdx     int          // index of next target waypoint
+	progress  float64      // 0..1 toward next waypoint
+	speed     float64      // progress increment per tick
+}
+
+func (s *shipState) tick() {
+	s.progress += s.speed
+	for s.progress >= 1 {
+		s.progress -= 1
+		s.wpIdx = (s.wpIdx + 1) % len(s.waypoints)
+	}
+	from := s.waypoints[(s.wpIdx-1+len(s.waypoints))%len(s.waypoints)]
+	to := s.waypoints[s.wpIdx]
+	s.lng = from[0] + (to[0]-from[0])*s.progress
+	s.lat = from[1] + (to[1]-from[1])*s.progress
+}
+
+// heading returns clockwise degrees from north (for SVG rotation).
+func (s *shipState) heading() float64 {
+	from := s.waypoints[(s.wpIdx-1+len(s.waypoints))%len(s.waypoints)]
+	to := s.waypoints[s.wpIdx]
+	dx := to[0] - from[0]
+	dy := to[1] - from[1]
+	// atan2 gives angle from +X axis; convert to CW from north
+	return math.Mod(math.Atan2(dx, dy)*180/math.Pi+360, 360)
+}
+
+var (
+	fleetOnce sync.Once
+	fleet     struct {
+		mu    sync.RWMutex
+		ships [3]shipState
+	}
+)
+
+const shipSVG = `<svg width="48" height="28" viewBox="0 0 80 44" xmlns="http://www.w3.org/2000/svg">` +
+	`<path d="M2 30 L10 42 L70 42 L78 30 Z" fill="#1b3a5c"/>` +
+	`<rect x="12" y="24" width="56" height="6" rx="1" fill="#2c5f8a"/>` +
+	`<rect x="18" y="14" width="46" height="10" rx="1" fill="#2c5f8a" stroke="#1b3a5c" stroke-width="0.5"/>` +
+	`<rect x="8" y="12" width="9" height="12" rx="1" fill="#d5dfe8" stroke="#2c5f8a" stroke-width="0.5"/>` +
+	`<rect x="9.5" y="13" width="6" height="4" rx="0.5" fill="#85c1e9"/>` +
+	`<rect x="10" y="4" width="5" height="8" rx="0.5" fill="#1b3a5c"/>` +
+	`<rect x="9.5" y="2" width="6" height="3" rx="0.5" fill="#c0392b"/>` +
+	`</svg>`
+
 func main() {
 	v := via.New()
 	v.Config(via.Options{
@@ -48,6 +99,50 @@ func main() {
 		}()
 	})
 
+	// Fleet of ships following waypoint loops through SF Bay.
+	fleetOnce.Do(func() {
+		fleet.ships = [3]shipState{
+			{ // Golden Gate → Alcatraz → Pier 39 → back out
+				waypoints: [][2]float64{
+					{-122.478, 37.819}, {-122.423, 37.827},
+					{-122.410, 37.809}, {-122.423, 37.827},
+				},
+				speed: 0.03,
+			},
+			{ // Oakland → Treasure Island → Angel Island → loop
+				waypoints: [][2]float64{
+					{-122.330, 37.795}, {-122.370, 37.823},
+					{-122.432, 37.860}, {-122.370, 37.823},
+				},
+				speed: 0.02,
+			},
+			{ // Sausalito → Pier 39 ferry route
+				waypoints: [][2]float64{
+					{-122.480, 37.859}, {-122.435, 37.840},
+					{-122.410, 37.809}, {-122.435, 37.840},
+				},
+				speed: 0.025,
+			},
+		}
+		// Set initial positions.
+		for i := range fleet.ships {
+			s := &fleet.ships[i]
+			s.lng = s.waypoints[0][0]
+			s.lat = s.waypoints[0][1]
+			s.wpIdx = 1
+		}
+		go func() {
+			for {
+				time.Sleep(time.Second)
+				fleet.mu.Lock()
+				for i := range fleet.ships {
+					fleet.ships[i].tick()
+				}
+				fleet.mu.Unlock()
+			}
+		}()
+	})
+
 	v.Page("/", func(c *via.Context) {
 		m := maplibre.New(c, maplibre.Options{
 			Style:  "https://demotiles.maplibre.org/style.json",
@@ -75,24 +170,27 @@ func main() {
 			},
 		})
 
-		// Custom SVG container ship marker (static)
-		m.AddMarker("ship", maplibre.Marker{
-			LngLat: maplibre.LngLat{Lng: -122.38, Lat: 37.80},
-			Element: `<svg width="48" height="28" viewBox="0 0 80 44" xmlns="http://www.w3.org/2000/svg">` +
-				`<path d="M2 30 L10 42 L70 42 L78 30 Z" fill="#1b3a5c"/>` +
-				`<rect x="12" y="24" width="56" height="6" rx="1" fill="#2c5f8a"/>` +
-				`<rect x="18" y="14" width="46" height="10" rx="1" fill="#2c5f8a" stroke="#1b3a5c" stroke-width="0.5"/>` +
-				`<rect x="8" y="12" width="9" height="12" rx="1" fill="#d5dfe8" stroke="#2c5f8a" stroke-width="0.5"/>` +
-				`<rect x="9.5" y="13" width="6" height="4" rx="0.5" fill="#85c1e9"/>` +
-				`<rect x="10" y="4" width="5" height="8" rx="0.5" fill="#1b3a5c"/>` +
-				`<rect x="9.5" y="2" width="6" height="3" rx="0.5" fill="#c0392b"/>` +
-				`</svg>`,
+		// Animated container ships following waypoint routes
+		shipNames := [3]string{"MSC Adriatica", "Evergreen Harmony", "Maersk Aurora"}
+		type shipSignals struct{ lng, lat *via.Signal }
+		var ships [3]shipSignals
+
+		fleet.mu.RLock()
+		for i, s := range fleet.ships {
+			ships[i].lng = c.Signal(s.lng)
+			ships[i].lat = c.Signal(s.lat)
+			m.AddMarker(fmt.Sprintf("ship-%d", i), maplibre.Marker{
+				LngSignal: ships[i].lng,
+				LatSignal: ships[i].lat,
+				Element:   shipSVG,
 				Anchor:    "center",
-			Rotation: 45,
+				Rotation:  s.heading(),
 				Popup: &maplibre.Popup{
-				Content: "<strong>MSC Adriatica</strong><p>Container vessel — heading NE</p>",
+					Content: fmt.Sprintf("<strong>%s</strong>", shipNames[i]),
 				},
 			})
+		}
+		fleet.mu.RUnlock()
 
 		// Custom SVG vehicle marker — reads shared Go state
 		vehicleLng := c.Signal(-122.43)
@@ -113,6 +211,14 @@ func main() {
 			vehicle.mu.RUnlock()
 			vehicleLng.SetValue(lng)
 			vehicleLat.SetValue(lat)
+
+			fleet.mu.RLock()
+			for i, s := range fleet.ships {
+				ships[i].lng.SetValue(s.lng)
+				ships[i].lat.SetValue(s.lat)
+			}
+			fleet.mu.RUnlock()
+
 			c.SyncSignals()
 		})