feat(sky): pack-gated procedural night sky replacing the stretched DAT star layer; register IA-26

User-directed enhancement ('I want the night sky to look very good'):
retail's star layer is one small texture stretched over a 10-poly dome
cap, so stars smear regardless of source-image quality. With the
Atmospheric render pack active, sky.frag now renders the star layer
(GfxObj 0x010015EF, identical in all 20 Dereth day groups) as a fully
procedural sky computed from the view direction: hash-derived stars on
a cube-face grid in three density tiers plus sparse diffraction-spiked
standouts, sized in SCREEN pixels via derivatives so they stay crisp at
any resolution and FOV, over the user-approved 0.4-1.3% cool mottle
(gen_starfield2.py seed 11, approved 2026-08-23). The draw is forced
additive; the day/night fade rides the star layer's existing retail
lighting product so the schedule matches the authored keyframes. Pack
inactive = retail look byte-untouched.

EnhancedNightSkyRuleTests pins the uParamA gate, the exact star-layer
id, the forced-additive draw, and the pack-runtime wiring; sky shader
SPIR-V recompiled and re-pinned. Hermetic App suite green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/Sky/SkyRenderer.Rhi.cs

@@ -209,7 +209,7 @@ public sealed unsafe partial class SkyRenderer
     /// those binds are what select the descriptor scope the sections must land in
     /// (plan §5.5.14 item 2).</para>
     /// </summary>
-    private void DrawSubMeshRhi(SubMeshGpu sub, uint textureSlot)
+    private void DrawSubMeshRhi(SubMeshGpu sub, uint textureSlot, bool nightSky = false)
     {
         if (sub.IndexCount == 0 || sub.VertexBuffer is null || sub.IndexBuffer is null)
             return;
@@ -220,12 +220,16 @@ public sealed unsafe partial class SkyRenderer
             ?? throw new InvalidOperationException(
                 "SkyRenderer requires an open IGpuFrame (see GpuDeviceFrameLifetime).");
 
-        encoder.BindPipeline(sub.IsAdditive ? _additivePipeline! : _alphaPipeline!);
+        // The enhanced night sky ADDS starlight over the dome, so its draw
+        // rides the additive pipeline even though the authored star surface
+        // is alpha-blended (register IA-26).
+        encoder.BindPipeline(nightSky || sub.IsAdditive ? _additivePipeline! : _alphaPipeline!);
         var pushConstants = new GpuPushConstants
         {
             // uViewProjection is unread by sky.vert — the sky carries its own
             // camera-anchored view and dome projection in SkyParams — so the
-            // block's only live member here is the texture slot.
+            // live members here are the texture slot and the night-sky
+            // ParamA/ParamB pair (sky.frag's uParamA gate + seed).
             ViewProjection = System.Numerics.Matrix4x4.Identity,
             DrawIdOffset = 0,
             LightingMode = 0,
@@ -233,8 +237,8 @@ public sealed unsafe partial class SkyRenderer
             LightDebug = 0,
             TextureIndexA = textureSlot,
             TextureIndexB = 0,
-            ParamA = 0f,
-            ParamB = 0f,
+            ParamA = nightSky ? 1f : 0f,
+            ParamB = NightSkySeed,
         };
         encoder.SetPushConstants(in pushConstants);
         encoder.BindVertexBuffer(0, sub.VertexBuffer, 0);

src/AcDream.App/Rendering/Sky/SkyRenderer.cs

@@ -89,6 +89,27 @@ public sealed partial class SkyRenderer : IDisposable
     public float Near { get; set; } = 0.1f;
     public float Far  { get; set; } = 1_000_000f;
 
+    /// <summary>
+    /// Dereth's star layer — the one sky GfxObj (verified identical across
+    /// all 20 day groups) whose draw the enhanced night sky replaces.
+    /// </summary>
+    private const uint StarLayerGfxObjId = 0x010015EFu;
+
+    /// <summary>
+    /// User-directed enhancement gate (register IA-26): when this returns
+    /// true, the star layer renders as sky.frag's procedural night sky
+    /// (view-direction stars, pixel-crisp at any resolution) instead of the
+    /// authored stretched texture. Wired by composition to "the Atmospheric
+    /// render pack is active"; null/false keeps the retail look. Evaluated
+    /// per draw — never capture a stale pre-session snapshot
+    /// (claude-memory/feedback_resolve_deferred_funcs_per_call.md).
+    /// </summary>
+    internal Func<bool>? EnhancedNightSkyActive { get; set; }
+
+    /// <summary>Deterministic seed for the procedural sky's hash grids —
+    /// the approved-look generator's seed (gen_starfield2.py, 2026-08-23).</summary>
+    private const float NightSkySeed = 11f;
+
     /// <summary>
     /// Draw all NON-WEATHER sky objects (dome, sun, moon, stars, clouds —
     /// every <c>SkyObject</c> with <c>Properties &amp; 0x04 == 0</c>).
@@ -426,7 +447,9 @@ public sealed partial class SkyRenderer : IDisposable
                     || obj.TexVelocityX != 0f
                     || obj.TexVelocityY != 0f;
                 uint slot = TextureTableSlot(sub.SurfaceId, needsRepeat);
-                DrawSubMeshRhi(sub, slot);
+                bool nightSky = gfxObjId == StarLayerGfxObjId
+                    && (EnhancedNightSkyActive?.Invoke() ?? false);
+                DrawSubMeshRhi(sub, slot, nightSky);
             }
         }
     }