refactor(weather): delete legacy camera-attached rain/snow particle emitter

The pre-research workaround at GameWindow.UpdateWeatherParticles +
BuildRainDesc + BuildSnowDesc was acdream's stand-in for retail's
weather rendering. It emitted billboarded particles inside a 15m disk
attached to the camera ('AttachLocal'), with a broken alpha fade
(0.3 → 0 caused rain to vanish at exact ground level — Issue #1) and a
fixed disk that visibly framed the player even at speed.

Retail rain is the world-space mesh path (SkyRenderer.RenderWeather):
  GfxObj 0x01004C42 / 0x01004C44 — hollow octagonal cylinder, 113m radius,
  815m tall, anchored at player_pos + (0, 0, -120m) per
  GameSky::UpdatePosition at 0x00506dd0 — drawn AFTER the landblock pass
  per LScape::draw at 0x00506330. Snow renders identically when a Snowy
  DayGroup is active: the partition by Properties&0x04 picks up snow
  weather meshes for free.

The legacy emitter was gated behind ACDREAM_FAKE_RAIN_PARTICLES=1 in
the previous commit (3e0da49) so the world-space path could be
A/B-compared. Visual verification this session confirmed the world-
space path is correct; deleting the legacy code removes ~120 LOC plus
the env var, the gate, the _rainEmitterHandle / _snowEmitterHandle
fields, and the _lastWeatherKind state machine.

Files affected:
  GameWindow.cs: drop UpdateWeatherParticles, BuildRainDesc, BuildSnowDesc,
  emitter-handle fields, last-weather-kind state, and the gated call site.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -373,12 +373,6 @@ public sealed class GameWindow : IDisposable
     private long _loadedSkyDayIndex = long.MinValue;
     private AcDream.Core.World.DayGroupData? _activeDayGroup;
 
-    // Current rain/snow emitter handles — spawned on weather-kind change
-    // and stopped when the kind leaves Rain/Snow. Non-zero == active.
-    private int _rainEmitterHandle;
-    private int _snowEmitterHandle;
-    private AcDream.Core.World.WeatherKind _lastWeatherKind =
-        AcDream.Core.World.WeatherKind.Clear;
     private double _weatherAccum;
 
     // F7 / F10 debug-cycle steps for time + weather. Initialized out of
@@ -4229,20 +4223,19 @@ public sealed class GameWindow : IDisposable
         Weather.Tick(nowSeconds: _weatherAccum, dayIndex: dayIndex, dtSeconds: (float)deltaSeconds);
         _weatherAccum += deltaSeconds;
 
-        // Update the rain/snow particle emitters when the weather kind
+        // (Pre-Bug-A code spawned camera-attached rain/snow particle
-        // changes. Keep the emitters fed by the ParticleSystem tick so
+        // emitters here as a workaround for missing weather-mesh
-        // visuals stay alive frame-over-frame.
+        // rendering. Deleted 2026-04-26 once the retail-faithful world-
-        //
+        // space mesh path landed in SkyRenderer.RenderWeather. Retail
-        // Bug A note (2026-04-26): retail rain is the world-space mesh
+        // rain is GfxObj 0x01004C42/0x01004C44 — a hollow octagonal
-        // 0x01004C42/0x01004C44 rendered in SkyRenderer.RenderWeather
+        // cylinder anchored at player_pos + (0, 0, -120m) per
-        // AFTER the scene — see docs/research/2026-04-26-sky-investigation-handoff.md.
+        // GameSky::UpdatePosition at 0x00506dd0 — drawn after the
-        // The camera-attached emitter path here is acdream's old
+        // landblock pass per LScape::draw at 0x00506330. There is no
-        // pre-research workaround (broken alpha fade, fixed disk around
+        // server-driven weather event and no camera-attached emitter
-        // camera). It's kept gated behind ACDREAM_FAKE_RAIN_PARTICLES=1
+        // in retail. Snow renders identically when a Snowy DayGroup is
-        // so the retail-faithful path can be A/B-tested against it
+        // active in some other Region; the partition by Properties&0x04
-        // without uninstalling the legacy code outright.
+        // and the SkyRenderer.RenderWeather pass both pick up snow
-        if (System.Environment.GetEnvironmentVariable("ACDREAM_FAKE_RAIN_PARTICLES") == "1")
+        // weather meshes for free.)
-            UpdateWeatherParticles(atmo);
 
         // Phase E.3: advance live particle emitters AFTER animation tick
         // so emitters spawned by hooks fired this frame get integrated.
@@ -5241,114 +5234,6 @@ public sealed class GameWindow : IDisposable
         }
     }
 
-    /// <summary>
-    /// Keep the rain/snow camera-anchored emitters aligned with the
-    /// current weather state. Spawns on entry, stops on exit, with no
-    /// per-frame churn while the state is stable. Emitters are camera-
-    /// local (<see cref="AcDream.Core.Vfx.EmitterFlags.AttachLocal"/>)
-    /// so walking never leaves the rain volume (r12 §7).
-    /// </summary>
-    private void UpdateWeatherParticles(in AcDream.Core.World.AtmosphereSnapshot atmo)
-    {
-        if (_particleSystem is null) return;
-
-        if (atmo.Kind == _lastWeatherKind) return;  // no change
-
-        // Stop any existing emitters first.
-        if (_rainEmitterHandle != 0)
-        {
-            _particleSystem.StopEmitter(_rainEmitterHandle, fadeOut: true);
-            _rainEmitterHandle = 0;
-        }
-        if (_snowEmitterHandle != 0)
-        {
-            _particleSystem.StopEmitter(_snowEmitterHandle, fadeOut: true);
-            _snowEmitterHandle = 0;
-        }
-
-        // Anchor at camera world position; AttachLocal keeps it moving.
-        var anchor = System.Numerics.Vector3.Zero;
-        if (_cameraController is not null)
-        {
-            System.Numerics.Matrix4x4.Invert(_cameraController.Active.View, out var inv);
-            anchor = new System.Numerics.Vector3(inv.M41, inv.M42, inv.M43);
-        }
-
-        switch (atmo.Kind)
-        {
-            case AcDream.Core.World.WeatherKind.Rain:
-            case AcDream.Core.World.WeatherKind.Storm:
-                _rainEmitterHandle = _particleSystem.SpawnEmitter(
-                    BuildRainDesc(), anchor);
-                break;
-            case AcDream.Core.World.WeatherKind.Snow:
-                _snowEmitterHandle = _particleSystem.SpawnEmitter(
-                    BuildSnowDesc(), anchor);
-                break;
-        }
-
-        _lastWeatherKind = atmo.Kind;
-    }
-
-    /// <summary>
-    /// Rain emitter tuned per r12 §7: streaks falling at ~50 m/s with
-    /// a slight wind bias, 500 drops/sec, 2000 max alive, 1.2s life so
-    /// drops cover the ~60m fall at terminal velocity.
-    /// </summary>
-    private static AcDream.Core.Vfx.EmitterDesc BuildRainDesc() => new()
-    {
-        DatId            = 0xFFFF_0001u,  // synthetic id
-        Type             = AcDream.Core.Vfx.ParticleType.LocalVelocity,
-        Flags            = AcDream.Core.Vfx.EmitterFlags.AttachLocal |
-                           AcDream.Core.Vfx.EmitterFlags.Billboard,
-        EmitRate         = 500f,
-        MaxParticles     = 2000,
-        LifetimeMin      = 1.0f,
-        LifetimeMax      = 1.4f,
-        OffsetDir        = new System.Numerics.Vector3(0, 0, 1),
-        MinOffset        = 0f,
-        MaxOffset        = 50f,
-        SpawnDiskRadius  = 15f,
-        InitialVelocity  = new System.Numerics.Vector3(0.5f, 0f, -50f),
-        VelocityJitter   = 2f,
-        Gravity          = System.Numerics.Vector3.Zero,
-        StartColorArgb   = 0x40B0C0E0u,
-        EndColorArgb     = 0x20B0C0E0u,
-        StartAlpha       = 0.3f,
-        EndAlpha         = 0f,
-        StartSize        = 0.05f,
-        EndSize          = 0.05f,
-    };
-
-    /// <summary>
-    /// Snow emitter tuned per r12 §8: slow fall at ~2 m/s, tumbling
-    /// sideways drift, small billboards, 100 flakes/sec, long lifespan.
-    /// </summary>
-    private static AcDream.Core.Vfx.EmitterDesc BuildSnowDesc() => new()
-    {
-        DatId            = 0xFFFF_0002u,
-        Type             = AcDream.Core.Vfx.ParticleType.LocalVelocity,
-        Flags            = AcDream.Core.Vfx.EmitterFlags.AttachLocal |
-                           AcDream.Core.Vfx.EmitterFlags.Billboard,
-        EmitRate         = 100f,
-        MaxParticles     = 1000,
-        LifetimeMin      = 4f,
-        LifetimeMax      = 8f,
-        OffsetDir        = new System.Numerics.Vector3(0, 0, 1),
-        MinOffset        = 0f,
-        MaxOffset        = 30f,
-        SpawnDiskRadius  = 15f,
-        InitialVelocity  = new System.Numerics.Vector3(0.3f, 0.2f, -2f),
-        VelocityJitter   = 0.8f,
-        Gravity          = System.Numerics.Vector3.Zero,
-        StartColorArgb   = 0xE0FFFFFFu,
-        EndColorArgb     = 0x80FFFFFFu,
-        StartAlpha       = 0.85f,
-        EndAlpha         = 0.3f,
-        StartSize        = 0.08f,
-        EndSize          = 0.06f,
-    };
-
     // ── Phase I.2 — DebugPanel helpers ────────────────────────────────
     //
     // The ImGui DebugPanel reads through DebugVM closures that ask