Mark the place the map is pointed at, and make following work

Choosing a town moved the camera and then left the reader to work out which of
the settlements now on screen was the one they asked for. A town has no precise
position, so the map now marks its centre: a white disc under a coloured dot,
drawn as a style layer so the native renderer keeps it pinned through every pan
and zoom. The radar frames are inserted below it, because a band of rain must
not paint over the one thing that says which town this is.

A searched municipality becomes a PointTarget rather than being thrown away as
"the whole region". It is marked, the recentre button returns to it, and the
chip names it - but it is never persisted, so looking something up no longer
costs the user the place their app opens on. That was a real defect: searching
went through select(FreeTarget), which wrote null over the stored preference.

Following the device was broken outright. MapLibre reports a camera animation
the app started exactly as it reports the user grabbing the map, so engaging
follow and then animating to the position cancelled the follow it had just
started; the resulting FreeTarget then re-framed the whole region. Follow now
moves the camera first and switches tracking on second, and FreeTarget no longer
moves the camera as a reaction to the state changing - framing the region is an
action, so panning away while following keeps the view the user panned to.

Verified on the emulator with a fix in Turin: a searched town is marked and
saveable, the app reopens on it with the marker in place, following centres on
the blue dot, and panning stops the chase without yanking the map away.

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

Nuvolari/docs/architecture.md

@@ -57,6 +57,40 @@ lib/
 Dependencies point inwards: `features` depends on `data`, `data` depends on `core`,
 `core` depends on nothing in the app. A feature never imports another feature.
 
+## What the map draws, and in what order
+
+Three things sit on the base map, and the order matters:
+
+```
+place marker  (GeoJSON source + two circle layers)   <- always on top
+radar frames  (two image layers, double buffered)
+base map      (OpenFreeMap vector tiles)
+```
+
+`PlaceMarker` creates its layers when the style finishes loading, and `RadarOverlay`
+inserts its frames *below* them with `addImageLayerBelow`. Order is not left to whichever
+attaches first: the radar attaches when the first frame arrives, which can be before or
+after the user picks a place, and a band of rain painted over the marker would hide the
+one thing on screen that says which town was chosen.
+
+The marker is a style layer rather than a widget in the `Stack` over the map. The native
+renderer keeps a layer pinned to its coordinates through every pan and zoom; a widget
+would need repositioning from Dart on each camera frame, one asynchronous coordinate
+conversion at a time, and would swim behind the map while it moved.
+
+### Following the device
+
+Two behaviours of MapLibre shape how `FollowUser` works:
+
+- **A camera animation the app starts is reported as tracking dismissed**, exactly like
+  the user grabbing the map. So follow is engaged by moving the camera *first* and
+  switching tracking on *second*. Done the other way round, the animation that brings the
+  camera to the user cancels the following it was meant to start.
+- **Dropping out of following lands in `FreeTarget`**, which is also the state the user
+  reaches by choosing "the whole region". So `FreeTarget` must never move the camera as a
+  reaction to the state changing — that would throw away the pan the user just made.
+  Framing the region is an action: the opening view, the menu entry, the recentre button.
+
 ## The adapter seam
 
 ```dart