polish(aprs): consolidate callsign regex, document raise, note negative-sample v1 limit

- Extract PathParser.valid_callsign?/1 and reuse from the calibration
  Mix task instead of duplicating the regex.
- Document raise behavior on Aprs.recent_packets_with_paths/1 and
  Aprs.station_positions/1 (already noted in moduledoc; @doc reinforces
  for callers reading the function head).
- Note in the BandConfig guard comment why it sits before the Oban pause
  (so a misconfig exits without leaving queues paused).
- Reword the band-mismatched-negatives note as a v1 limitation (no TODO
  tag — codebase convention is zero TODOs).

lib/microwaveprop/aprs.ex

@@ -71,6 +71,7 @@ defmodule Microwaveprop.Aprs do
   Returns recent position-bearing packets with non-empty paths, oldest first.
 
   Filters out item/object packets and rows missing position or path.
+  Raises on connection or SQL failure — see moduledoc on async-context safety.
 
   ## Options
 
@@ -96,6 +97,9 @@ defmodule Microwaveprop.Aprs do
   Returns the most recent known position for each callsign in `callsigns`.
 
   Callsigns with no positioned packets simply don't appear in the result.
+  Empty input list short-circuits to `%{}` without querying.
+
+  Raises on connection or SQL failure — see moduledoc on async-context safety.
   """
   @spec station_positions([String.t()]) :: %{String.t() => position()}
   def station_positions([]), do: %{}

lib/microwaveprop/aprs/path_parser.ex

@@ -43,6 +43,15 @@ defmodule Microwaveprop.Aprs.PathParser do
   # rather than `qA[A-Za-z]` which would over-match `qAa`, `qAd`, …
   @q_constructs ~w(qAC qAX qAU qAo qAO qAS qAr qAR qAZ qAI)
 
+  @doc """
+  Returns `true` if `token` matches the APRS callsign shape used by this
+  parser (1-2 alpha/digit prefix, one digit, 1-3 alpha suffix, optional
+  -SSID 0..99). Exposed so callers (e.g. the calibration Mix task) can
+  pre-filter path tokens with the same rule the parser itself applies.
+  """
+  @spec valid_callsign?(String.t()) :: boolean()
+  def valid_callsign?(token) when is_binary(token), do: Regex.match?(@callsign_regex, token)
+
   @spec parse_hops(
           sender_callsign :: String.t(),
           sender_pos :: position(),

lib/mix/tasks/calibrate.aprs_144.ex

@@ -35,7 +35,6 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
   alias Microwaveprop.Weather
 
   @factor_keys ~w(humidity time_of_day td_depression refractivity sky season wind rain pwat pressure)a
-  @callsign_regex ~r/^[A-Z0-9]{1,2}[0-9][A-Z]{1,3}(-[0-9]{1,2})?$/
   @min_samples 50
   @band_mhz 144
 
@@ -43,6 +42,8 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
   def run(argv) do
     Mix.Task.run("app.start")
 
+    # Guard runs BEFORE the Oban pause so a misconfig exits without
+    # leaving queues paused. Subsequent failures are inside try/after.
     if BandConfig.get(@band_mhz) == nil do
       Mix.raise("BandConfig has no #{@band_mhz} MHz entry; cannot calibrate.")
     end
@@ -132,7 +133,7 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
       |> Enum.map(&String.trim/1)
       |> Enum.map(&String.trim_trailing(&1, "*"))
     end)
-    |> Enum.filter(&Regex.match?(@callsign_regex, &1))
+    |> Enum.filter(&PathParser.valid_callsign?/1)
     |> Enum.uniq()
   end
 
@@ -169,6 +170,13 @@ defmodule Mix.Tasks.Calibrate.Aprs144 do
     # 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.
+    #
+    # v1 limitation: negatives sample from the full contacts table (all
+    # bands, dominantly 10 GHz tropo). For a 144 MHz fit this means the
+    # trainer separates "verified VHF receive" from "anywhere a 10 GHz
+    # contact happened with timestamp jitter". A future iteration should
+    # draw negatives from APRS coverage where no `*` digi handled the
+    # frame in the same window — true band-matched negatives.
     baselines = Backtest.random_baseline(n, sample_size: max(n, 5_000))
 
     factors =