fix(weather): prune dense .sgrid scalar files

ScalarFile.list_valid_time_dirs/1 stripped only a `.hrdps` suffix
before DateTime.from_iso8601/1, so `<iso>.sgrid` and
`<iso>.hrdps.sgrid` never parsed and were skipped by both
prune_older_than/1 and retain_window/2. Since .sgrid is the format the
pipeline actually writes, nothing on the dense tier was ever deleted —
production held 222 files spanning 8 days against a 48h window.

Strip the known suffixes in order, mirroring ProfilesFile's anchored
parser (which documents this exact failure mode for .hrdps.prop).
Orphaned .tmp.* writes still don't parse, so they stay with the tmp
sweep in Propagation.prune_old_scores/0.

Also corrects the CLAUDE.md entry that recorded this as already fixed —
the call site was present, the suffix parsing was not.

CLAUDE.md

@@ -263,7 +263,7 @@ Both `.pgrid` (profiles) and `.sgrid` (weather scalars) use this `write_atomic`
 
 **Known cleanup gaps (confirmed):**
 - `.hrdps.prop` score files — ✅ **Confirmed fixed**: `scores_file.ex:516` `parse_valid_time_dt` handles `.hrdps.prop` via `~r/^(.+)\.(?:hrdps\.prop|prop|ntms)$/`
-- `ScalarFile.prune_older_than` — ✅ **Confirmed fixed**: called from `propagation.ex:240` alongside `ScoresFile` and `ProfilesFile` prune calls
+- `ScalarFile.prune_older_than` — ✅ **Confirmed fixed**: called from `propagation.ex:240` alongside `ScoresFile` and `ProfilesFile` prune calls. Note the call site was never the problem — until 2026-08-05 `list_valid_time_dirs/1` stripped only a `.hrdps` suffix before `DateTime.from_iso8601/1`, so `<iso>.sgrid` never parsed and both prune paths silently skipped every dense file (production held 222 files spanning 8 days against a 48 h window). Verify suffix parsing, not just the call site, when auditing a prune path.
 - Orphaned `.tmp.*` files — ✅ **Confirmed fixed**: `propagation.ex:254` sweeps `ScoresFile.base_dir/ProfilesFile.base_dir/ScalarFile.base_dir` for `.tmp.*` files after each prune cycle
 
 **Known cleanup gaps (unverified):**

lib/microwaveprop/weather/scalar_file.ex

@@ -553,8 +553,8 @@ defmodule Microwaveprop.Weather.ScalarFile do
     case File.ls(base) do
       {:ok, entries} ->
         for entry <- entries,
-            iso = String.replace_suffix(entry, ".hrdps", ""),
-            {:ok, dt, _} <- [DateTime.from_iso8601(iso)],
+            dt = entry_valid_time(entry),
+            dt != nil,
             do: {Path.join(base, entry), dt}
 
       _ ->
@@ -562,6 +562,33 @@ defmodule Microwaveprop.Weather.ScalarFile do
     end
   end
 
+  # Entry shapes living under `weather_scalars/`:
+  #
+  #     <iso>              legacy HRRR chunk dir
+  #     <iso>.hrdps        legacy HRDPS chunk dir
+  #     <iso>.sgrid        dense HRRR scalars (current)
+  #     <iso>.hrdps.sgrid  dense HRDPS scalars (current)
+  #
+  # Strip the known suffixes in order, mirroring `ProfilesFile`'s
+  # anchored-extension parser. Handling only `.hrdps` left every
+  # `<iso>.sgrid` failing `DateTime.from_iso8601/1`, which made the
+  # dense files — the format the pipeline actually writes — invisible to
+  # `prune_older_than/1` and `retain_window/2`.
+  #
+  # Anything else (notably orphaned `.tmp.*` writes) returns nil and is
+  # left for the tmp sweep in `Propagation.prune_old_scores/0`.
+  defp entry_valid_time(entry) do
+    iso =
+      entry
+      |> String.replace_suffix(".sgrid", "")
+      |> String.replace_suffix(".hrdps", "")
+
+    case DateTime.from_iso8601(iso) do
+      {:ok, dt, _} -> dt
+      _ -> nil
+    end
+  end
+
   # Match ProfilesFile's snap step (0.125°) so a click at any lat/lon
   # rounds to the same key the writer used.
   defp snap(value) do

test/microwaveprop/weather/scalar_file_test.exs

@@ -461,5 +461,55 @@ defmodule Microwaveprop.Weather.ScalarFileTest do
       assert ScalarFile.exists?(inside)
       refute ScalarFile.exists?(outside)
     end
+
+    # `.sgrid` is the format the pipeline actually writes. Stripping only
+    # a `.hrdps` suffix before `from_iso8601/1` left every dense file
+    # unparseable and therefore invisible to both prune paths —
+    # production accumulated 222 files spanning 8 days against a 48 h
+    # retention window.
+    test "prune_older_than removes dense .sgrid files" do
+      old = ~U[2026-04-27 12:00:00Z]
+      new = ~U[2026-04-28 12:00:00Z]
+
+      File.mkdir_p!(ScalarFile.base_dir())
+      File.write!(Sgrid.path_for(old), "x")
+      File.write!(Sgrid.path_for_hrdps(old), "x")
+      File.write!(Sgrid.path_for(new), "x")
+
+      assert ScalarFile.prune_older_than(~U[2026-04-28 00:00:00Z]) == 2
+      refute File.exists?(Sgrid.path_for(old))
+      refute File.exists?(Sgrid.path_for_hrdps(old))
+      assert File.exists?(Sgrid.path_for(new))
+    end
+
+    test "retain_window removes dense .sgrid files outside the window" do
+      run_time = ~U[2026-04-28 12:00:00Z]
+      inside = ~U[2026-04-28 14:00:00Z]
+      outside = ~U[2026-04-29 06:00:00Z]
+
+      File.mkdir_p!(ScalarFile.base_dir())
+      File.write!(Sgrid.path_for(run_time), "x")
+      File.write!(Sgrid.path_for_hrdps(inside), "x")
+      File.write!(Sgrid.path_for(outside), "x")
+
+      assert ScalarFile.retain_window(run_time, 4) == 1
+      assert File.exists?(Sgrid.path_for(run_time))
+      assert File.exists?(Sgrid.path_for_hrdps(inside))
+      refute File.exists?(Sgrid.path_for(outside))
+    end
+
+    # Orphaned atomic-write temporaries are swept separately by
+    # `Propagation.prune_old_scores/0`; they must not parse as a
+    # valid_time here or a crashed write would be pruned as if it were
+    # a finished artifact.
+    test "ignores orphaned .tmp files" do
+      vt = ~U[2026-04-27 12:00:00Z]
+      File.mkdir_p!(ScalarFile.base_dir())
+      tmp = Sgrid.path_for(vt) <> ".tmp.123.4"
+      File.write!(tmp, "x")
+
+      assert ScalarFile.prune_older_than(~U[2026-04-28 00:00:00Z]) == 0
+      assert File.exists?(tmp)
+    end
   end
 end