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>

tests/AcDream.App.Tests/Rendering/Gpu/Vk/VulkanShaderManifestTests.cs

@@ -52,8 +52,8 @@ public sealed class VulkanShaderManifestTests
             // override, retail's GameSky::Draw @0x00506FF0 rule (see
             // SkyFogRuleTests). A deliberate default-path change, reviewed
             // with the world-fog-range fix in the same commit.
-            ["sky.frag.spv"] = "b3b544829f2dd85be04b6b16d78a0490e1fe7884590cb541b95756b7d7511620",
-            ["sky.vert.spv"] = "77176cf33c761ee4e9730357895c941dbf5949d8e0d28e0bb0dcde87f4d30288",
+            ["sky.frag.spv"] = "8105984072fc1b9075b5efdffd75d959c4087710d354d50a0e00ee0500462ca7",
+            ["sky.vert.spv"] = "3b51945fa4ff1be1604144df92866bdd47aade22f9dd90267591ef36adb28cde",
             ["terrain_modern.frag.spv"] = "7b3cdb01b837ed77ee20559a81c1ce5c9d5395300efcc072560ab0be3c5a1af9",
             ["terrain_modern.vert.spv"] = "9f4cb221ea6aed94a8d23af6cb8e3f3ed96c3cce6e50d135a72d3b55667b1557",
             ["ui_text.frag.spv"] = "37a281bf80441cb425eaa3ad8e0b3a43cfa21b74b60973ed4201718b9dc102df",

tests/AcDream.App.Tests/Rendering/Sky/EnhancedNightSkyRuleTests.cs

@@ -0,0 +1,106 @@
+using System;
+using System.IO;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering.Sky;
+
+/// <summary>
+/// Source guards for the enhanced night sky (register IA-26, user-directed
+/// 2026-08-23): a render-pack-gated replacement of the DAT star layer
+/// (GfxObj 0x010015EF) with sky.frag's procedural view-direction starfield.
+/// The retail look must stay the default: everything here is inert unless the
+/// Atmospheric pack is the active runtime. The shader itself needs a GPU, so
+/// these pin the wiring the way SkyFogRuleTests pins the fog rules.
+/// </summary>
+public sealed class EnhancedNightSkyRuleTests
+{
+    [Fact]
+    public void SkyFragmentShaderGatesTheProceduralSkyOnParamA()
+    {
+        string frag = File.ReadAllText(Path.Combine(ShaderRoot(), "sky.frag"));
+        string code = StripLineComments(frag);
+
+        Assert.Contains("if (uParamA > 0.5)", code, StringComparison.Ordinal);
+        Assert.Contains("nightSky(normalize(vDir), uint(uParamB))", code, StringComparison.Ordinal);
+        // Screen-pixel star sizing is the reason this exists — the stretched
+        // texture flaw must not creep back in via a fixed-size grid.
+        Assert.Contains("fwidth(g)", code, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SkyRendererSwapsExactlyTheStarLayerAndOnlyUnderTheProvider()
+    {
+        string source = File.ReadAllText(Path.Combine(
+            RepositoryRoot(), "src", "AcDream.App", "Rendering", "Sky", "SkyRenderer.cs"));
+        string code = StripLineComments(source);
+
+        Assert.Contains(
+            "private const uint StarLayerGfxObjId = 0x010015EFu;",
+            code,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "bool nightSky = gfxObjId == StarLayerGfxObjId",
+            code,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "(EnhancedNightSkyActive?.Invoke() ?? false)",
+            code,
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void NightSkyDrawForcesTheAdditivePipeline()
+    {
+        string source = File.ReadAllText(Path.Combine(
+            RepositoryRoot(), "src", "AcDream.App", "Rendering", "Sky", "SkyRenderer.Rhi.cs"));
+        string code = StripLineComments(source);
+
+        Assert.Contains(
+            "nightSky || sub.IsAdditive ? _additivePipeline! : _alphaPipeline!",
+            code,
+            StringComparison.Ordinal);
+        Assert.Contains("ParamA = nightSky ? 1f : 0f,", code, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CompositionGatesTheNightSkyOnTheAtmosphericPackRuntime()
+    {
+        string source = File.ReadAllText(Path.Combine(
+            RepositoryRoot(), "src", "AcDream.App", "Composition", "FrameRootComposition.cs"));
+        string code = StripLineComments(source);
+
+        Assert.Contains("EnhancedNightSkyActive = () =>", code, StringComparison.Ordinal);
+        Assert.Contains(
+            "nightSkyController.ActiveRuntime?.Descriptor.Id",
+            code,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "BuiltInAtmosphericRenderPack.Id",
+            code,
+            StringComparison.Ordinal);
+    }
+
+    private static string StripLineComments(string source)
+    {
+        var lines = source.Split('\n');
+        for (int i = 0; i < lines.Length; i++)
+        {
+            int idx = lines[i].IndexOf("//", StringComparison.Ordinal);
+            if (idx >= 0)
+                lines[i] = lines[i][..idx];
+        }
+        return string.Join('\n', lines);
+    }
+
+    private static string ShaderRoot() =>
+        Path.Combine(RepositoryRoot(), "src", "AcDream.App", "Rendering", "Shaders");
+
+    private static string RepositoryRoot()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "AcDream.slnx")))
+            directory = directory.Parent;
+        return directory?.FullName
+            ?? throw new InvalidOperationException("Could not locate repository root.");
+    }
+}