feat(weather): ?layer= URL param to deep-link a specific overlay

Adds a `layer` query parameter to `/weather`. Loading
`/weather?layer=temp_700mb` boots straight into that overlay; clicking
a different layer pill push_patches the URL so the active selection is
shareable and survives a refresh. Unknown layer ids fall back to the
default (temperature) instead of crashing.

lib/microwaveprop_web/live/weather_map_live.ex

@@ -160,6 +160,9 @@ defmodule MicrowavepropWeb.WeatherMapLive do
     }
   ]
 
+  @default_layer "temperature"
+  @valid_layer_ids Enum.map(@layers, & &1.id)
+
   @impl true
   def mount(_params, _session, socket) do
     _ =
@@ -187,7 +190,7 @@ defmodule MicrowavepropWeb.WeatherMapLive do
      assign(socket,
        page_title: "Weather Map",
        layers: @layers,
-       selected_layer: "temperature",
+       selected_layer: @default_layer,
        initial_data_json: "[]",
        valid_time: initial_vt,
        valid_times: valid_times,
@@ -201,6 +204,16 @@ defmodule MicrowavepropWeb.WeatherMapLive do
      )}
   end
 
+  @impl true
+  def handle_params(params, _uri, socket) do
+    selected = pick_layer(params["layer"], socket.assigns[:selected_layer])
+    {:noreply, assign(socket, :selected_layer, selected)}
+  end
+
+  defp pick_layer(layer, _current) when layer in @valid_layer_ids, do: layer
+  defp pick_layer(_layer, current) when is_binary(current), do: current
+  defp pick_layer(_layer, _current), do: @default_layer
+
   defp pick_initial_valid_time([]), do: nil
 
   defp pick_initial_valid_time(valid_times) do
@@ -210,7 +223,11 @@ defmodule MicrowavepropWeb.WeatherMapLive do
 
   @impl true
   def handle_event("select_layer", %{"layer" => layer_id}, socket) do
-    {:noreply, assign(socket, :selected_layer, layer_id)}
+    if layer_id in @valid_layer_ids do
+      {:noreply, push_patch(socket, to: ~p"/weather?layer=#{layer_id}")}
+    else
+      {:noreply, socket}
+    end
   end
 
   def handle_event("toggle_grid", _params, socket) do

test/microwaveprop_web/live/weather_map_live_test.exs

@@ -163,6 +163,32 @@ defmodule MicrowavepropWeb.WeatherMapLiveTest do
       assert html =~ "Minimum refractivity gradient"
       assert before =~ "Minimum refractivity gradient"
     end
+
+    test "updates the URL via push_patch so the active layer is shareable", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/weather")
+
+      render_hook(lv, "select_layer", %{"layer" => "pwat"})
+
+      assert_patched(lv, ~p"/weather?layer=pwat")
+    end
+  end
+
+  describe "?layer= URL parameter" do
+    test "pre-selects the requested layer on initial load", %{conn: conn} do
+      {:ok, _lv, html} = live(conn, ~p"/weather?layer=pwat")
+
+      assert html =~ ~s(data-selected-layer="pwat")
+      # PWAT description appears in the rendered sidebar (not just the
+      # JSON-encoded layers attribute), confirming the layer assign was
+      # actually applied.
+      assert html =~ "rain fade risk"
+    end
+
+    test "ignores an unknown layer id and falls back to the default", %{conn: conn} do
+      {:ok, _lv, html} = live(conn, ~p"/weather?layer=bogus_xyz")
+
+      assert html =~ ~s(data-selected-layer="temperature")
+    end
   end
 
   describe "toggle_grid + toggle_radar events" do