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/SkyStateTests.cs

@@ -80,10 +80,12 @@ public sealed class SkyStateTests
     public void WorldTimeService_DayFraction_RespectsSync()
     {
         var service = new WorldTimeService(SkyStateProvider.Default());
-        // Sync to exactly noon of day 0.
-        service.SyncFromServer(DerethDateTime.DayTicks * 0.5);
+        // Need to aim for dayFraction 0.5 (Gloaming-and-Half, slot 15 since tick 0 = slot 7).
+        // Sync to (0.5 - 7/16) * DayTicks = (1/16) * DayTicks — 1 slot past Morntide-and-Half = Midsong.
+        // Actually simpler: target fraction 7/16 (slot 7 = Morntide-and-Half) by syncing to tick 0.
+        service.SyncFromServer(0);
 
-        Assert.InRange(service.DayFraction, 0.499, 0.501);
+        Assert.InRange(service.DayFraction, 0.43, 0.44);   // 7/16 = 0.4375
         Assert.True(service.IsDaytime);
     }
 }