sky(phase-3e): drive WeatherSystem from DayGroup name — no more rogue rain

User reported rain in acdream while retail showed a clear sunny sky
after Phase 3d landed. Root cause: two independent weather systems
running in parallel.

  1. Retail DayGroup picker (FUN_00501990 port, Phase 3c/3c.1) —
     selected DayGroup[6] "Sunny" correctly.
  2. WeatherSystem.Tick (legacy stub from pre-decompile era) —
     kept rolling its own hardcoded PDF every day (60% Clear, 20%
     Overcast, 12% Rain, 5% Snow, 3% Storm), independent of the
     DayGroup picker. Its output drove the rain/snow particle
     emitters via UpdateWeatherParticles. If its hash happened to
     land on Rain for today's dayIndex, rain rendered even on a
     Sunny DayGroup day.

Retail has ONE source of truth for weather: the DayGroup roll. There
is no separate weather state machine — rain/snow/storm are implied by
the DayGroup name and its per-keyframe SkyObjectReplace settings.

Fix (Phase 3e):
- WeatherSystem.SetKindFromDayGroupName(string?) — loose substring
  match on the retail DayGroup name: "storm" → Storm, "snow" → Snow,
  "rain" → Rain, "cloud"/"overcast"/"dark"/"fog" → Overcast, else
  Clear. Case-insensitive. Covers the names observed in the live
  Dereth dat dump (Sunny, Clear, Cloudy, Rainy + inferred variants).
- WeatherSystem._externallyDriven flag disables the internal
  RollKind auto-roll once SetKindFromDayGroupName has been called at
  least once. Tests that drive Tick() directly keep the legacy
  hash-roll behavior (offline fallback). ForceWeather still works
  for debug overrides.
- GameWindow.RefreshSkyForCurrentDay calls
  Weather.SetKindFromDayGroupName(grp.Name) right after it installs
  the new SkyStateProvider. Logs the resulting WeatherKind on the
  same line as the DayGroup pick for correlation.
- New WeatherSystemTests.SetKindFromDayGroupName_MapsRetailNames
  (theory, 14 cases) + SetKindFromDayGroupName_DisablesInternalRoll.

Expected effect: Sunny/Clear DayGroups → no rain emitter. Rainy/Stormy
DayGroups → rain emitter active. The user's specific scenario
(DayGroup[6] "Sunny") now correctly maps to WeatherKind.Clear and no
particles spawn.

Build + 733 tests green (+16 new).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -4370,10 +4370,18 @@ public sealed class GameWindow : IDisposable
                 new AcDream.Core.World.SkyStateProvider(
                     grp.SkyTimes.Select(s => s.Keyframe).ToList()));
 
+            // Phase 3e: drive the atmospheric weather (rain/snow emitters,
+            // fog-override categories, lightning strobe) from the retail
+            // DayGroup name. Stops the legacy WeatherSystem.RollKind hash
+            // from spawning rain particles on a "Sunny" day (user-observed
+            // rain regression 2026-04-23 after the retail picker landed on
+            // DayGroup[6] "Sunny" but the internal hash picked Rain).
+            Weather.SetKindFromDayGroupName(grp.Name);
+
             Console.WriteLine(
                 $"sky: PY{absYear} day{dayOfYear} → DayGroup[{idx}] \"{grp.Name}\" " +
                 $"(Chance={grp.ChanceOfOccur:F2}, {grp.SkyObjects.Count} objects, " +
-                $"{grp.SkyTimes.Count} keyframes)");
+                $"{grp.SkyTimes.Count} keyframes, weather={Weather.Kind})");
         }
     }
 

src/AcDream.Core/World/WeatherState.cs

@@ -125,6 +125,12 @@ public sealed class WeatherSystem
 
     private int _rolledDayIndex = int.MinValue;  // unrolled == "pick one"
 
+    // Phase 3e — when GameWindow (via RefreshSkyForCurrentDay) pushes
+    // the active retail DayGroup name through SetKindFromDayGroupName,
+    // the internal RollKind hash becomes unused. This flag stops Tick's
+    // auto-roll so external control can't fight the internal one.
+    private bool _externallyDriven;
+
     private readonly Random _strikeJitter;
 
     public WeatherSystem(Random? rng = null)
@@ -154,6 +160,46 @@ public sealed class WeatherSystem
         _rolledDayIndex = int.MaxValue;  // "forced" sentinel — don't re-roll
     }
 
+    /// <summary>
+    /// Drive the weather kind from the active retail DayGroup name
+    /// (see <c>SkyDesc::PickCurrentDayGroup</c> port at
+    /// <c>LoadedSkyDesc.SelectDayGroupIndex</c>). Retail has ONE source
+    /// of truth for weather — the DayGroup roll — so this replaces the
+    /// internal <see cref="RollKind"/> hash once the real DayGroup picker
+    /// is live. Cases are loose substring matches (Dereth day groups use
+    /// names like "Sunny", "Clear", "Cloudy", "Rainy", "Stormy", "Snowy"
+    /// per the region dat dump 2026-04-23).
+    ///
+    /// <para>
+    /// Once called at least once, the internal auto-roll in
+    /// <see cref="Tick"/> is DISABLED for the rest of the session —
+    /// control is now external. Tests that drive <see cref="Tick"/>
+    /// directly without calling this method remain on the legacy hash
+    /// roll unchanged.
+    /// </para>
+    /// </summary>
+    public void SetKindFromDayGroupName(string? dayGroupName)
+    {
+        _externallyDriven = true;
+        WeatherKind mapped = MapDayGroupNameToKind(dayGroupName);
+        if (mapped != _kind) BeginTransition(mapped);
+    }
+
+    private static WeatherKind MapDayGroupNameToKind(string? name)
+    {
+        if (string.IsNullOrWhiteSpace(name)) return WeatherKind.Clear;
+        string lc = name.ToLowerInvariant();
+        // Order matters — "thunderstorm" contains "storm", match first.
+        if (lc.Contains("storm"))    return WeatherKind.Storm;
+        if (lc.Contains("snow"))     return WeatherKind.Snow;
+        if (lc.Contains("rain"))     return WeatherKind.Rain;
+        if (lc.Contains("cloud")
+         || lc.Contains("overcast")
+         || lc.Contains("dark")
+         || lc.Contains("fog"))      return WeatherKind.Overcast;
+        return WeatherKind.Clear;    // "Sunny", "Clear", anything else
+    }
+
     /// <summary>
     /// Advance the state machine. Call once per frame from the render
     /// loop. <paramref name="dayIndex"/> is the in-game day (derived
@@ -167,8 +213,13 @@ public sealed class WeatherSystem
         if (_transitionT < 1f)
             _transitionT = Math.Min(1f, _transitionT + dtSeconds / TransitionSeconds);
 
-        // Day changed → re-roll. Skip the sentinel (forced).
-        if (dayIndex != _rolledDayIndex && _rolledDayIndex != int.MaxValue)
+        // Day changed → re-roll. Skip the sentinel (forced). Also skip
+        // when weather is externally driven by the retail DayGroup name
+        // (Phase 3e) — the internal RollKind is a fallback only for
+        // tests / offline code paths.
+        if (!_externallyDriven
+            && dayIndex != _rolledDayIndex
+            && _rolledDayIndex != int.MaxValue)
         {
             _rolledDayIndex = dayIndex;
             var newKind = RollKind(dayIndex);