weather(phase-5d): AdminEnvirons packet handler + thunder sound dispatch

Ports retail's AdminEnvirons (opcode 0xEA60) — the client-visible
weather-event channel distinct from the PlayScript path. Wire format
(chunk_006A0000.c: `[u32 opcode][u32 environChangeType]`).

EnvironChangeType range:
  0x00..0x06 — fog presets (Clear/Red/Blue/White/Green/Black/Black2)
  0x65..0x75 — one-shot ambient sounds (Roar, Bell, Chant, etc)
  0x76..0x7B — Thunder1..6 sounds (paired with a lightning PlayScript)

Dispatch:
  - WorldSession decodes the packet, fires EnvironChanged event.
  - GameWindow.OnEnvironChanged:
    * Fog values (0x00..0x06) → WeatherSystem.Override. The enum
      values line up byte-for-byte with our EnvironOverride enum
      (deliberately mirrored from retail), so a direct cast works.
    * Sound values (0x65..0x7B) → console log with retail name for
      now. Actual OpenAL playback needs a EnvironChangeType →
      WaveData lookup (indexed via SoundTable dat), which is a
      separate follow-up. The event still fires so any future
      audio subscriber can plug in.

Combined with Phase 6a-6c PhysicsScript/PlayScript wiring, the
complete retail lightning pipeline is now:

  server sends PlayScript(0xF754, lightningGuid, scriptId=0x33xxxxxx)
    → runs the flash script via PhysicsScriptRunner
    → CreateParticleHook spawns the flash particles
  server sends AdminEnvirons(0xEA60, Thunder3Sound=0x78)
    → OnEnvironChanged logs; audio binding TBD

Whether the user's ACE sends these packets depends on the server
(ACE 2.x vanilla does NOT — Agent #5 verified no lightning opcodes in
the default emit path). With the client port complete, any ACE mod
or extension that emits the right packets will Just Work in acdream.

Build + 742 tests green.

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

src/AcDream.Core.Net/WorldSession.cs

@@ -132,6 +132,33 @@ public sealed class WorldSession : IDisposable
     /// </summary>
     public event Action<uint /*guid*/, uint /*scriptId*/>? PlayScriptReceived;
 
+    /// <summary>
+    /// Phase 5d — retail's <c>AdminEnvirons</c> packet (opcode
+    /// <c>0xEA60</c>) — the one-and-only channel retail's server uses
+    /// for weather environment changes. Wire format:
+    /// <c>[u32 opcode][u32 environChangeType]</c>. The payload enum is
+    /// retail's <c>EnvironChangeType</c>:
+    /// <list type="bullet">
+    ///   <item><description>
+    ///     <c>0x00..0x06</c> — fog presets (Clear/Red/Blue/White/Green/
+    ///     Black/Black2). Subscribers route these to a
+    ///     <see cref="AcDream.Core.World.WeatherSystem.Override"/>.
+    ///   </description></item>
+    ///   <item><description>
+    ///     <c>0x65..0x75</c> — one-shot ambient sound cues
+    ///     (Roar / Bell / Chant / etc).
+    ///   </description></item>
+    ///   <item><description>
+    ///     <c>0x76..0x7B</c> — Thunder1..Thunder6 sounds. Paired with
+    ///     a separate <see cref="PlayScriptReceived"/> from the server
+    ///     carrying the lightning-flash PhysicsScript.
+    ///   </description></item>
+    /// </list>
+    /// See <c>docs/research/2026-04-23-lightning-crossfade.md</c> +
+    /// <c>2026-04-23-lightning-real.md</c>.
+    /// </summary>
+    public event Action<uint /*environChangeType*/>? EnvironChanged;
+
     /// <summary>
     /// Phase G.1: latest server Portal Year tick count. Seeded from the
     /// ConnectRequest handshake (r12 §1.3 — server sends absolute game
@@ -571,6 +598,19 @@ public sealed class WorldSession : IDisposable
                 var env = GameEventEnvelope.TryParse(body);
                 if (env is not null) GameEvents.Dispatch(env.Value);
             }
+            else if (op == 0xEA60u)  // AdminEnvirons — server pushes a fog preset or sound cue
+            {
+                // Phase 5d: wire format `[u32 opcode][u32 environChangeType]`
+                // per chunk_006A0000.c. Dispatch the event; GameWindow
+                // subscribers route fog presets into WeatherSystem.Override
+                // and sound cues (thunder, roar, etc) into the audio engine.
+                if (body.Length >= 8)
+                {
+                    uint envType = System.Buffers.Binary.BinaryPrimitives
+                        .ReadUInt32LittleEndian(body.AsSpan(4, 4));
+                    EnvironChanged?.Invoke(envType);
+                }
+            }
             else if (op == 0xF754u)  // PlayScript — server triggers a PhysicsScript on a target guid
             {
                 // Phase 6b: wire format `[u32 opcode][u32 guid][u32 scriptId]`