fix(world): DerethDateTime tick-0 offset — sky was 7/16 of a day wrong

User observed: 'time is flipped — supposed to be day/evening, but shows
night/morning.' That's a ~half-day offset.

Root cause in ACE DerethDateTime.cs line 23:
  private const double dayZeroTicks = 0; // Morningthaw 1, 10 P.Y. - Morntide-and-Half

ACE anchors tick 0 to Morntide-and-Half (slot 7 on the 0-indexed 16-slot
scale) — NOT Darktide (slot 0 = midnight) as our DayFraction function
assumed. Confirmed by DerethDateTime.cs:145:
  private int hour = (int)Hours.Morntide_and_Half;

Fix: shift DayFraction by +7/16 * DayTicks (3333.75) so tick 0 maps to
its real calendar slot. Exposed as DayFractionOriginOffsetTicks constant
for documentation + downstream referencing.

Effect on sun: previously, server tick ~0 (just-booted ACE) produced
dayFraction 0 → midnight sky → night colors at noon real-time.
Now dayFraction 7/16 = 0.4375 → late morning sky → noon-ish colors
within 1/16 of a day, which matches what a user actually sees when
launching during daytime.

Tests updated for the corrected convention:
- DerethDateTime.DayFraction(0) = 7/16 (not 0).
- CurrentHour(0) = MorntideAndHalf (not Darktide).
- IsDaytime(0) = true.
- Midnight (Darktide, slot 0) is 9/16 of a day past tick 0.
- SkyState + WorldTimeDebug tests retargeted to the new frame.

Build green, 711 tests pass.

Ref: references/ACE/Source/ACE.Common/DerethDateTime.cs:23-25 + :145.

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

tests/AcDream.Core.Tests/World/WorldTimeDebugTests.cs

@@ -10,21 +10,32 @@ public sealed class WorldTimeDebugTests
     public void SetDebugTime_OverridesDayFraction()
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
-        service.SyncFromServer(0);  // midnight
+        service.SyncFromServer(0);  // server tick 0 (= Morntide-and-Half)
 
-        service.SetDebugTime(0.5f);   // force noon
+        service.SetDebugTime(0.5f);   // force noon (Midsong-and-Half)
         Assert.InRange(service.DayFraction, 0.499, 0.501);
     }
 
     [Fact]
     public void ClearDebugTime_RestoresServerTime()
     {
+        // Post tick-0-offset fix: DayFraction(tick) = ((tick + 7/16 * DayTicks) % DayTicks) / DayTicks.
+        // Pick a server tick whose real-world meaning is straightforward to verify.
+        // Sync to (0.25 - 7/16) * DayTicks negative means "3 slots before midnight
+        // past Morntide-and-Half", which in positive terms is 13/16 of the day
+        // past Morntide-and-Half, but simpler: sync to "1/16 past midnight" =
+        // ticks giving fraction 1/16. Required tick offset from 0 to land at
+        // fraction 1/16: solve (t + 7/16*D) mod D = 1/16*D
+        //                 → t = (1/16 - 7/16) * D mod D = -6/16 * D mod D = 10/16 * D.
+        double targetFraction = 1.0 / 16.0;  // Darktide-and-Half
+        double syncTick = (targetFraction - (7.0 / 16.0) + 1.0) * DerethDateTime.DayTicks;
+
         var service = new WorldTimeService(SkyStateProvider.Default());
-        service.SyncFromServer(DerethDateTime.DayTicks * 0.25); // dawn
+        service.SyncFromServer(syncTick);
         service.SetDebugTime(0.5f);
         service.ClearDebugTime();
 
-        Assert.InRange(service.DayFraction, 0.24, 0.26);
+        Assert.InRange(service.DayFraction, targetFraction - 0.01, targetFraction + 0.01);
     }
 
     [Fact]
@@ -32,9 +43,9 @@ public sealed class WorldTimeDebugTests
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
         service.SetDebugTime(0.75f);
-        service.SyncFromServer(0);  // midnight — this should clear the override
+        service.SyncFromServer(0);  // tick 0 = Morntide-and-Half → fraction 7/16
 
-        Assert.InRange(service.DayFraction, 0.0, 0.01);
+        Assert.InRange(service.DayFraction, 7.0 / 16.0 - 0.01, 7.0 / 16.0 + 0.01);
     }
 
     [Fact]