sky(phase-3f): anchor calendar to dat's GameTime.ZeroTimeOfYear

Final piece of the retail-sync puzzle. Live Dereth dat has
GameTime.ZeroTimeOfYear = 3600 (verified 2026-04-23 diagnostic dump).
Our DerethDateTime hardcoded +7/16 × DayTicks = 3333.75, copied from
ACE's DerethDateTime.cs comment "tick 0 = Morntide-and-Half". The dat
is authoritative; ACE's comment is wrong by 266.25 ticks (~33 Dereth
minutes).

User-observed regression (2026-04-23):
  acdream: middle-of-night (Darktide), clear, DayGroup "Sunny"
  retail:  near-pre-dawn (Foredawn), thunderstorm, stormy DayGroup
  (both connected to the same ACE at PortalYearTicks=291134079)

Same server tick → different calendar extraction → the offset skewed
dayFraction AND pushed DayOfYear across a boundary at certain ticks,
feeding a different LCG seed into the DayGroup picker (FUN_00501990).
A single 266.25-tick offset error explains both the time mismatch and
the weather mismatch.

Code changes:
- DerethDateTime.OriginOffsetTicks — runtime-settable static, default
  = DayFractionOriginOffsetTicks (3333.75, the legacy fallback).
  Applied in DayFraction, Year, DayOfYear, ToCalendar.
- DerethDateTime.SetOriginOffsetFromDat(double) — called at Region
  load.
- SkyDescLoader.DumpRegionSkyDesc dumps GameTime fields (and all 16
  TimesOfDay entries) when ACDREAM_DUMP_SKY=1.
- GameWindow.LoadRegion adopts the dat's ZeroTimeOfYear after
  LoadFromRegion, logs the before/after values.

Also dumps every Dereth TimeOfDay hour-boundary (0..15) so any future
calendar weirdness has authoritative ground truth in the log.

Build + 733 tests green (no test depended on the hardcoded offset).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -893,6 +893,26 @@ public sealed class GameWindow : IDisposable
                 // 2026-04-23).
                 WorldTime.TickSize = 1.0;
 
+                // Phase 3f: adopt the dat's GameTime.ZeroTimeOfYear as the
+                // calendar-extraction offset. Dereth's dat value is 3600
+                // (verified 2026-04-23 live dump); ACE's DerethDateTime.cs
+                // comment that "tick 0 = Morntide-and-Half" (3333.75
+                // offset = +7/16) is WRONG by 266.25 ticks against the
+                // authoritative dat. The mismatch cascaded into both the
+                // wrong hour label AND the wrong DayOfYear at boundary
+                // times (different LCG seed → different DayGroup roll),
+                // which explained the user's observation of "acdream
+                // clear night, retail stormy pre-dawn" at the same
+                // server PortalYearTicks.
+                if (region.GameTime is not null)
+                {
+                    AcDream.Core.World.DerethDateTime.SetOriginOffsetFromDat(
+                        region.GameTime.ZeroTimeOfYear);
+                    Console.WriteLine(
+                        $"sky: GameTime ZeroTimeOfYear={region.GameTime.ZeroTimeOfYear} " +
+                        $"(was default {AcDream.Core.World.DerethDateTime.DayFractionOriginOffsetTicks})");
+                }
+
                 Console.WriteLine(
                     $"sky: loaded Region 0x13000000 — {_loadedSkyDesc.DayGroups.Count} day groups, " +
                     $"SkyDesc.TickSize={_loadedSkyDesc.TickSize} (throttle, not rate), " +