refactor(aprs): mix calibrate.aprs_144 surfaces silent drops + guards

- Print '<computed> / <attempted>' for both positive (HRRR-coverage) and
  negative (random-baseline → HRRR) factor compute stages so an operator
  can see how much of the corpus survived each filter.
- Guard at task entry: Mix.raise if BandConfig has no 144 MHz entry.
  Without it the Recalibrator silently falls back to 10 GHz physics.
- random_baseline now draws from sample_size: max(n, 5_000) so small-n
  fits don't cluster geographically.
- Wrap body in try/after Oban.resume_all_queues so iex -S mix workflows
  don't leave Oban paused after the task returns.

lib/mix/tasks/calibrate.aprs_144.ex

@@ -42,8 +42,24 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
   @impl Mix.Task
   def run(argv) do
     Mix.Task.run("app.start")
+
+    if BandConfig.get(@band_mhz) == nil do
+      Mix.raise("BandConfig has no #{@band_mhz} MHz entry; cannot calibrate.")
+    end
+
     _ = Oban.pause_all_queues(Oban)
 
+    try do
+      do_run(argv)
+    after
+      # Allow `iex -S mix` workflows to keep using Oban after the task
+      # returns; for one-shot Mix invocations the BEAM exits and this is
+      # a no-op.
+      _ = Oban.resume_all_queues(Oban)
+    end
+  end
+
+  defp do_run(argv) do
     {opts, _, _} =
       OptionParser.parse(argv,
         switches: [
@@ -60,10 +76,10 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
     max_packets = Keyword.get(opts, :max_packets, 50_000)
 
     Mix.shell().info("APRS-144 calibration")
-    Mix.shell().info("  since_hours:  #{since_hours}")
-    Mix.shell().info("  epochs:       #{epochs}")
+    Mix.shell().info("  since_hours:   #{since_hours}")
+    Mix.shell().info("  epochs:        #{epochs}")
     Mix.shell().info("  learning_rate: #{learning_rate}")
-    Mix.shell().info("  max_packets:  #{max_packets}")
+    Mix.shell().info("  max_packets:   #{max_packets}")
     Mix.shell().info("")
 
     since = DateTime.add(DateTime.utc_now(), -since_hours * 3600, :second)
@@ -79,14 +95,22 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
     Mix.shell().info("Parsed #{length(hops)} verified RF hops")
 
     positives = compute_positive_factors(hops)
-    Mix.shell().info("Computed #{length(positives)} positive factor vectors with HRRR coverage")
+
+    Mix.shell().info(
+      "Computed #{length(positives)} / #{length(hops)} positive factor vectors " <>
+        "(#{length(hops) - length(positives)} hops dropped for missing HRRR coverage)"
+    )
 
     if length(positives) < @min_samples do
       Mix.shell().info("")
       Mix.shell().info("Refusing to fit: only #{length(positives)} positive samples (need >= #{@min_samples})")
     else
-      negatives = compute_negative_factors(length(positives))
-      Mix.shell().info("Computed #{length(negatives)} negative factor vectors")
+      {negatives, attempted} = compute_negative_factors(length(positives))
+
+      Mix.shell().info(
+        "Computed #{length(negatives)} / #{attempted} negative factor vectors " <>
+          "(#{attempted - length(negatives)} samples dropped for missing HRRR coverage)"
+      )
 
       if length(negatives) < @min_samples do
         Mix.shell().info("")
@@ -142,14 +166,20 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
   end
 
   defp compute_negative_factors(n) do
-    n
-    |> Backtest.random_baseline(sample_size: n)
-    |> Enum.flat_map(fn {lat, lon, time} ->
-      case Weather.find_nearest_hrrr(lat, lon, time) do
-        nil -> []
-        profile -> [Recalibrator.compute_factors(profile, time, @band_mhz)]
-      end
-    end)
+    # Use a wide contact-pool draw (5_000 minimum) so the random baseline
+    # samples don't cluster geographically when n is small. Backtest's
+    # default :sample_size is 5_000.
+    baselines = Backtest.random_baseline(n, sample_size: max(n, 5_000))
+
+    factors =
+      Enum.flat_map(baselines, fn {lat, lon, time} ->
+        case Weather.find_nearest_hrrr(lat, lon, time) do
+          nil -> []
+          profile -> [Recalibrator.compute_factors(profile, time, @band_mhz)]
+        end
+      end)
+
+    {factors, length(baselines)}
   end
 
   defp print_results(result, positives, negatives) do