sky(phase-3c.1): feed AbsoluteYear (Year+ZeroYear) to retail LCG picker

Live verification (2026-04-23, Phase 3c launch): acdream picked
DayGroup[17] "Rainy" for PY106 day46 while retail at the same server
tick showed clear blue sky with white clouds (Sunny-ish). Root cause:
our port passed the RELATIVE year (106, i.e. years since tick-0) into
the LCG seed, while retail's TimeOfDay+0x64 is ABSOLUTE Year =
floor(...) + ZeroYear (baseYear=10 for Dereth GameTime). The offset
seeds the LCG with `seed = 106×7620+46` vs retail's `seed =
116×7620+46` — `10 × SecondsPerDay = 76200` apart, guaranteed to
land on a different DayGroup index.

Fix:
- DerethDateTime.ZeroYear constant (= 10) + AbsoluteYear(ticks) helper.
- GameWindow.RefreshSkyForCurrentDay feeds AbsoluteYear into the picker.
- LoadedSkyDesc.ActiveDayGroup(ticks) same.
- Calendar display and generic Year(ticks) stay relative; only the
  LCG-seed path uses the offset. Matches retail FUN_005a7510:5300 which
  explicitly adds baseYear to the relative year before stashing in
  TimeOfDay+0x64.

Build + 717 tests green. Next visual check should show matching
weather with retail client side-by-side.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/World/DerethDateTime.cs

@@ -57,6 +57,15 @@ public static class DerethDateTime
     /// </summary>
     public const double MaxTicks = 1_073_741_828.0;
 
+    /// <summary>
+    /// Base/anchor year for the Portal Year calendar (retail
+    /// <c>GameTime.ZeroYear</c> = 10). Tick 0 corresponds to PY 10,
+    /// Morningthaw 1, Morntide-and-Half. Retail's <c>TimeOfDay+0x64</c>
+    /// ("absolute year") includes this offset, and <c>SkyDesc::PickCurrentDayGroup</c>
+    /// (<c>FUN_00501990</c>) feeds the absolute year into its LCG seed.
+    /// </summary>
+    public const int ZeroYear = 10;
+
     /// <summary>
     /// The 16 named hour slots (r12 §1.2). Each one is half a Derethian
     /// hour; the "-and-Half" variants are the second half.
@@ -178,13 +187,11 @@ public static class DerethDateTime
     }
 
     /// <summary>
-    /// Absolute year since tick-0 (PY 0, Snowreap 1, Morntide-and-Half).
-    /// Matches retail's <c>TimeOfDay + 0x64</c> field which is
-    /// <c>floor((worldTime + base) / secsPerYear) + baseYear</c>. For
-    /// acdream's purposes we treat ZeroYear=0 since only the RELATIVE
-    /// year is needed for deterministic day-group rolling (retail and
-    /// acdream agree as long as both apply the same derivation to the
-    /// server's PortalYearTicks).
+    /// Year offset from tick-0 (number of Derethian years elapsed since
+    /// the PY <see cref="ZeroYear"/> anchor). Use for time-delta math.
+    /// For calendar display and for the retail
+    /// <c>SkyDesc::PickCurrentDayGroup</c> LCG seed, use
+    /// <see cref="AbsoluteYear"/> which adds the <c>ZeroYear</c> base.
     /// </summary>
     public static int Year(double ticks)
     {
@@ -192,6 +199,17 @@ public static class DerethDateTime
         return (int)(ticks / YearTicks);
     }
 
+    /// <summary>
+    /// Absolute Portal Year (<see cref="ZeroYear"/> + relative year).
+    /// Matches retail's <c>TimeOfDay + 0x64</c> field
+    /// (<c>FUN_005a7510:5300</c>:
+    /// <c>floor((worldTime+base)/secsPerYear) + baseYear</c>). This is
+    /// the value the retail DayGroup picker (<c>FUN_00501990</c>) feeds
+    /// into its LCG seed, so acdream must match for identical weather
+    /// picks vs retail.
+    /// </summary>
+    public static int AbsoluteYear(double ticks) => Year(ticks) + ZeroYear;
+
     /// <summary>
     /// Day index within the current Derethian year in [0, 359] (360 days
     /// per year = 12 months × 30). Matches retail's <c>TimeOfDay + 0x68</c>

src/AcDream.Core/World/SkyDescLoader.cs

@@ -235,10 +235,10 @@ public sealed class LoadedSkyDesc
     /// </summary>
     public DayGroupData? ActiveDayGroup(double serverTicks)
     {
-        int year = DerethDateTime.Year(serverTicks);
+        int absYear = DerethDateTime.AbsoluteYear(serverTicks);
         int dayOfYear = DerethDateTime.DayOfYear(serverTicks);
         int secondsPerDay = (int)DerethDateTime.DayTicks;    // 7620
-        int idx = SelectDayGroupIndex(year, secondsPerDay, dayOfYear);
+        int idx = SelectDayGroupIndex(absYear, secondsPerDay, dayOfYear);
         return idx < DayGroups.Count ? DayGroups[idx] : null;
     }