Add waypoint route line, snap-to-edit, and halo action menu

Full reticle interaction state machine (idle/snapped/editing), matching
the HTML prototype's mode variable:

- idle: reticle shows "+", tap drops a new waypoint at the map center.
- snapped: after any pan ends, if the map center lands within the
  reticle's on-screen radius (converted to meters at the current
  zoom/latitude) of an existing waypoint, the view auto-recenters onto it
  and the reticle switches to a pencil icon. Tap to enter editing.
- editing: reticle shows a checkmark; panning the map now live-updates the
  target waypoint's position (drag-to-reposition) instead of moving the
  camera "away" from it, and the HaloMenu radial menu appears.

HaloMenu: three ring-wedge hit regions (custom ClipPath/CustomClipper,
angles/radii taken from the prototype's buildRingWedgePath) for
Loiter/Landing/Remove, with Material icons standing in for the
demonstrator's hand-drawn SVGs. Loiter/Landing toggle the waypoint's
action and drop back to snapped; Remove deletes it and returns to idle.

MissionMap now draws a straight white polyline connecting all waypoints
in order (the physically-grounded straight-line + fillet-arc geometry
from doc 3.7/4.6 is still open - this is a straight-line placeholder) plus
a dashed rubber-band preview from the last waypoint to the live map center
while idle.

Note on porting from Leaflet: flutter_map's MapController.move() emits
MapEventMove, not MapEventMoveEnd, so (unlike the prototype's panTo()) the
auto-recenter-on-snap doesn't re-trigger snap detection - no "autoPanning"
guard flag was needed.

Verified with flutter analyze, flutter test (three new tests: halo menu
open/close, remove, loiter toggle - the wedge tap tests had to target the
icon's exact center via tapAt(), since a wedge's bounding-box center falls
outside its actual donut-segment hit area), and a full manual pass on the
Pixel_10a emulator: idle -> add -> snap (auto-recenter confirmed) -> edit
-> drag-reposition (confirmed via re-snap after further panning showing
the moved position, not the original one) -> halo remove -> back to idle,
plus the connecting line rendering correctly between two waypoints set
far apart.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/ui/widgets/reticle_button.dart

@@ -1,12 +1,14 @@
 import 'package:flutter/material.dart';
 
-/// Zentrales Steuerelement zum Setzen von Wegpunkten (HTML-Demonstrator:
-/// #centerCluster/#reticleBtn), fixiert in der Bildschirmmitte. Die Karte
-/// wird darunter verschoben; ein Tap setzt einen Wegpunkt an der aktuellen
-/// Kartenmitte.
+/// Zentrales Steuerelement zum Setzen/Editieren von Wegpunkten
+/// (HTML-Demonstrator: #centerCluster/#reticleBtn), fixiert in der
+/// Bildschirmmitte. Die Karte wird darunter verschoben. Icon wechselt je
+/// nach Modus: "+" (idle, neuer Wegpunkt), Stift (snapped, zum Bearbeiten
+/// antippen), Haken (editing, bestaetigen).
 class ReticleButton extends StatelessWidget {
-  const ReticleButton({super.key, required this.onTap});
+  const ReticleButton({super.key, required this.icon, required this.onTap});
 
+  final IconData icon;
   final VoidCallback onTap;
 
   @override
@@ -21,7 +23,7 @@ class ReticleButton extends StatelessWidget {
           color: Colors.black.withValues(alpha: 0.68),
           border: Border.all(color: Colors.white, width: 2),
         ),
-        child: const Icon(Icons.add, color: Colors.white, size: 26),
+        child: Icon(icon, color: Colors.white, size: 26),
       ),
     );
   }