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/Shaders/spv/shaders.manifest.json

@@ -311,12 +311,12 @@
       "stages": [
         {
           "stage": "vert",
-          "sourceSha256": "a89580f54c8d36b33d50b84b9fb61024861c737bb0b5bd7a4df0704bf010c3c3",
+          "sourceSha256": "9102b156bd4fd667831353640b2feee2ddde66e88579a4e425566c9b67304b64",
           "compiled": true
         },
         {
           "stage": "frag",
-          "sourceSha256": "b1f3b924af2040c909337c02461ed6795b5320fb04b5bb371ce1546feead567c",
+          "sourceSha256": "bf5bc353959636b1c80a9f72dc918cffdd33fb6b7b655149de40422030b4f51a",
           "compiled": true
         }
       ]