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/Composition/FrameRootComposition.cs

@@ -320,6 +320,20 @@ internal sealed class FrameRootCompositionPhase
                     renderPackDiagnostics.BindOwned(renderPackController));
             }
             atmosphericInputs = new AcDream.App.Rendering.Packs.AtmosphericFrameInputState();
+
+            // Enhanced night sky (register IA-26, user-directed 2026-08-23):
+            // while the Atmospheric pack is the active runtime, SkyRenderer
+            // swaps the DAT star layer for sky.frag's procedural
+            // view-direction starfield. Same-composition lifetimes on both
+            // sides, and the delegate re-reads ActiveRuntime per draw, so a
+            // pack activate/deactivate flips the sky the same frame.
+            if (live.SkyRenderer is { } skyForNightSky)
+            {
+                var nightSkyController = renderPackController;
+                skyForNightSky.EnhancedNightSkyActive = () =>
+                    nightSkyController.ActiveRuntime?.Descriptor.Id
+                        == AcDream.App.Rendering.Packs.BuiltInAtmosphericRenderPack.Id;
+            }
         }
 
         var renderWeatherFrame = new RenderWeatherFrameController(