feat: crowd flow visualization implementation

Implements comprehensive crowd flow visualization for the Spaxel
WiFi CSI-based indoor positioning system. The feature tracks movement
patterns over time and renders them as directional flow maps and dwell
hotspot heatmaps.

Key components:
- FlowAccumulator: Subscribes to TrackManager updates (10 Hz) and
  accumulates trajectory data with 0.2m sampling threshold
- SQLite tables: trajectory_segments, dwell_accumulator, detected_corridors
- Flow map computation: Bresenham's line algorithm for grid traversal,
  5-minute cache, time/person filtering
- Dwell heatmap: Stationary detection (< 0.1 m/s), normalized 0-1
- Corridor detection: Circular variance analysis, connected components
- 3D visualization: Animated arrows (flow), heatmap patches (dwell),
  raised platforms (corridors)
- REST API: /api/analytics/flow, /api/analytics/dwell, /api/analytics/corridors
- Dashboard controls: Time/person filters, layer toggles, auto-refresh

Background tasks:
- Daily pruning of trajectory segments older than 90 days
- Weekly corridor detection and recomputation

Load shedding integration:
- Suspends crowd flow accumulation at Level 1 (≥80ms iteration time)
- Respects ShouldAccumulateCrowdFlow() flag from loadshed package

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.needle-predispatch-sha

@@ -1 +1 @@
-bbb29a2629d9adb853c2924c688bf3d5f65d22b4
+a97960bf671844b7cdea739580979225d8a8cd73

mothership/internal/fleet/handler_test.go

@@ -513,7 +513,7 @@ func TestHandlerListFleet(t *testing.T) {
 			sendIdentifyFunc: func(mac string, durationMS int) bool {
 				return true
 			},
-			getConnectedMACs: func() []string {
+			GetConnectedMACs: func() []string {
 				return []string{"AA:BB:CC:DD:EE:FF"}
 			},
 		},

mothership/internal/fleet/registry.go

@@ -12,6 +12,18 @@ import (
 	_ "modernc.org/sqlite"
 )
 
+// NodeRegistry is the interface for node registry operations.
+// This allows both the real Registry and mock implementations to be used interchangeably.
+type NodeRegistry interface {
+	GetNode(mac string) (*NodeRecord, error)
+	GetAllNodes() ([]NodeRecord, error)
+	SetNodeLabel(mac, label string) error
+	SetNodePosition(mac string, x, y, z float64) error
+	AddVirtualNode(mac, name string, x, y, z float64) error
+	DeleteNode(mac string) error
+	SetRoom(room RoomConfig) error
+}
+
 // NodeRecord stores persistent node metadata.
 type NodeRecord struct {
 	MAC             string