feat(rover): add ModeThresholds for SSB/CW/Q65 SNR table

lib/microwaveprop/propagation/mode_thresholds.ex

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+defmodule Microwaveprop.Propagation.ModeThresholds do
+  @moduledoc """
+  Required-SNR thresholds (dB) for the modes the rover planner supports.
+
+  The rover predicts a per-link SNR margin; subtracting the mode's required
+  SNR yields "dB above decode threshold". Numbers come from the rover spec —
+  SSB needs full quieting, CW pulls signals well below the noise floor, and
+  Q65 modes go deeper still.
+  """
+
+  @thresholds %{
+    ssb: 0.0,
+    cw: -18.0,
+    q65_60a: -24.0,
+    q65_120b: -26.0
+  }
+
+  @modes [
+    {:ssb, "SSB"},
+    {:cw, "CW"},
+    {:q65_60a, "Q65-60A"},
+    {:q65_120b, "Q65-120B"}
+  ]
+
+  @type mode :: :ssb | :cw | :q65_60a | :q65_120b
+
+  @spec required_snr_db(mode()) :: float()
+  def required_snr_db(mode) when is_map_key(@thresholds, mode), do: Map.fetch!(@thresholds, mode)
+
+  @spec modes() :: [{mode(), String.t()}]
+  def modes, do: @modes
+
+  @spec default_mode() :: mode()
+  def default_mode, do: :ssb
+end

test/microwaveprop/propagation/mode_thresholds_test.exs

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+defmodule Microwaveprop.Propagation.ModeThresholdsTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Propagation.ModeThresholds
+
+  describe "required_snr_db/1" do
+    test "returns the documented thresholds for each supported mode" do
+      assert ModeThresholds.required_snr_db(:ssb) == 0.0
+      assert ModeThresholds.required_snr_db(:cw) == -18.0
+      assert ModeThresholds.required_snr_db(:q65_60a) == -24.0
+      assert ModeThresholds.required_snr_db(:q65_120b) == -26.0
+    end
+
+    test "raises FunctionClauseError for unknown modes" do
+      assert_raise FunctionClauseError, fn -> ModeThresholds.required_snr_db(:fm) end
+    end
+  end
+
+  describe "modes/0" do
+    test "returns ordered {atom, label} pairs covering every threshold" do
+      modes = ModeThresholds.modes()
+      assert {:ssb, "SSB"} in modes
+      assert {:cw, "CW"} in modes
+      assert {:q65_60a, "Q65-60A"} in modes
+      assert {:q65_120b, "Q65-120B"} in modes
+      assert length(modes) == 4
+    end
+  end
+
+  describe "default_mode/0" do
+    test "returns the SSB atom" do
+      assert ModeThresholds.default_mode() == :ssb
+    end
+  end
+end