sky(phase-8): retail-faithful night sky + README refresh

Iteration on the sky rendering pipeline to restore stars/moon visibility
at night and fix washed-out grey daytime clouds. Key fixes:

* sky.frag: disable fog-mix on sky meshes. Retail's keyframe FogEnd
  (0..400m at midnight, up to 2400m during day) is calibrated for
  terrain; sky meshes are authored at radii 1050-14271m which sits
  past FogEnd universally, causing every sky pixel to saturate to
  fogColor (dark navy). Stars, moon, dome texture all got
  obliterated. The horizon-glow trade-off is noted in the shader
  comment; research item to find retail's sky-specific fog range
  later.

* SkyRenderer + sky.frag: promote rep.Luminosity into uEmissive so the
  vertex lighting saturates properly for bright keyframes. Retail's
  FUN_0059da60 non-luminous path writes rep.Luminosity into
  material.Emissive via the cache +0x3c slot; we were instead using
  it as a post-fragment multiply which could only dim, never brighten.
  Net effect: daytime clouds now render saturated white, dome dims
  correctly at night (rep.Luminosity=0.11 → Emissive=0.11), stars
  and moon unchanged.

* terrain.vert: MIN_FACTOR 0.08 -> 0.0 per retail FUN_00532440 decompile
  (DAT_00796344 ambient-floor = 0.0). Back-lit terrain now falls to
  pure ambient rather than getting an 8% sun floor.

New research / tooling (no runtime impact):

* docs/research/2026-04-24-lambert-brightness-split.md — retail's
  ambient-brightness formula pinned from PE .rdata read + live
  RetailTimeProbe capture: effAmbBright = AmbBright + |sunDir| * 0.2
  where scale constant 0x0079a1e8 = 0.2f exactly.

* docs/research/2026-04-23-lightning-real.md — research note on the
  dat-baked PhysicsScript-driven lightning path (Rainy DayGroup has
  explicit PES-triggered flash SkyObjects with 5ms time windows).

* Corrections stapled to sky-decompile-hunt-{B,C}.md: DAT_00842778 is
  DirColor, DAT_0084277c is AmbColor (the hunt docs had the swap
  backwards).

* tools/RetailTimeProbe/Program.cs: extended with pid=NNNN selector,
  sky global probe (DirColor/AmbColor/AmbBright/sunDir/cache.amb),
  and the 0x0079a1e8 scale-factor readout.

* tools/SkyObjectInspect/: throwaway dat-inspector built by the Opus
  deep-dive agent. Identified GfxObj 0x010015EF as the stars layer
  (A8R8G8B8 128x128 texture, 4% bright-pixel ratio).

* src/AcDream.App/Rendering/TextureCache.cs: per-texture alpha
  histogram dump under ACDREAM_DUMP_SKY=1 for diagnosing "are the
  clouds decoded with proper alpha" type questions.

README: rewrite to reflect current state (playable pre-alpha rendering
Dereth with animated characters, day-night cycle, weather, etc.)
instead of the stale "Phase 0 dat inventory only" description.

All 742 tests green.

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

@@ -205,14 +205,18 @@ public sealed unsafe class SkyRenderer : IDisposable
                 else
                     _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
 
-                // Per-submesh emissive (Surface.Luminosity FLOAT field —
-                // 1.0 for dome + sun + moon, 0.0 for clouds). The vertex
-                // shader saturates the lighting math when emissive=1.0 so
-                // self-illuminated meshes render at full texture brightness
-                // regardless of time of day; emissive=0.0 meshes get the
-                // full `ambient + diffuse × sun` tint (producing retail's
-                // purple night clouds / warm dusk clouds / pale noon clouds).
-                _shader.SetFloat("uEmissive", sub.SurfLuminosity);
+                // Emissive source: retail's FUN_0059da60 for non-luminous
+                // surfaces writes rep.Luminosity into D3DMATERIAL9.Emissive
+                // (via material cache +0x3c). This PROMOTES bright-keyframe
+                // clouds into the self-lit term so the litColor saturates
+                // and the texture renders at full brightness rather than
+                // being dimmed by a per-fragment multiply.
+                //
+                // If no rep.Luminosity override: fall back to the Surface's
+                // static Luminosity (1.0 for dome/sun/moon → saturates;
+                // 0.0 for stars → stays ambient-lit, correct retail look).
+                float effEmissive = (luminosity > 0f) ? luminosity : sub.SurfLuminosity;
+                _shader.SetFloat("uEmissive", effEmissive);
 
                 uint tex = _textures.GetOrUpload(sub.SurfaceId);
                 _gl.ActiveTexture(TextureUnit.Texture0);