feat(world): Phase G.1 DerethDateTime + SkyStateProvider + WorldTimeService

Client-side deterministic sky + weather + day/night system per R12.
Retail's model is 95% client-side: the server just delivers its
current PortalYearTicks (double, seconds since boot-seed) at login and
in TimeSync packets; the client computes everything else locally from
the constants in r12 §1.2 + ACE DerethDateTime.cs.

Core layer (AcDream.Core/World):
- DerethDateTime: retail-exact calendar (16 hours/day, 30 days/month,
  12 months/year, 7620 ticks/day, 2,743,200 ticks/year). HourName enum
  covers all 16 named half-hour slots (Darktide → GloamingAndHalf);
  MonthName covers the 12 Derethian months (Snowreap → Frostfell).
  DayFraction, CurrentHour, IsDaytime, ToCalendar.
- SkyKeyframe + SkyStateProvider: 4-keyframe default day/dawn/noon/dusk
  with linear color + angular-wrap heading interpolation + slerp-like
  shortest-arc lerp so heading wraps 350° → 10° don't tween backwards
  through 180°. Default keyframe colors tuned to retail screenshots
  (sunrise warm, noon white, sunset red, midnight deep blue).
- WorldTimeService: owns the live clock. SyncFromServer(ticks) sets
  baseline; NowTicks advances by real-time elapsed. Exposes DayFraction,
  CurrentSky, CurrentSunDirection, IsDaytime for the render thread.

This is the foundation Phase G.2 (dynamic lighting) consumes: lighting
uniforms are fed from CurrentSky's SunColor / AmbientColor / sun
direction, varying smoothly across the day.

Tests (16 new):
- DerethDateTime: midnight, half-day, wrap, Dawnsong, Midsong,
  day/night flag at dawn vs Darktide-Half, year rollover, month
  advance.
- SkyState: 4-default keyframes, noon-exact matches frame data,
  midpoint lerps between neighbours, wrap across midnight doesn't
  produce NaN, sun direction returns unit vector, WorldTimeService
  sync + DayFraction at noon.

Build green, 587 tests pass (up from 570).

Ref: r12 §1 (Portal Year math), §2 (sky objects), §4 (color lerp).
Ref: ACE DerethDateTime.cs + NetworkSession TimeSync handler.

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

src/AcDream.Core/World/DerethDateTime.cs

@@ -0,0 +1,155 @@
+using System;
+
+namespace AcDream.Core.World;
+
+/// <summary>
+/// Asheron's Call "Portal Year" calendar + time-of-day math — faithful
+/// port of retail's <c>DerethDateTime</c> (r12 §1.2 + ACE
+/// <c>DerethDateTime.cs</c>).
+///
+/// <para>
+/// The server transports an absolute tick count as a double (seconds
+/// since a seed chosen at boot). The client doesn't need to know which
+/// seed was chosen — it just needs the tick count to derive:
+/// <list type="bullet">
+///   <item><description>
+///     Day fraction in [0, 1): where we are in the 16-hour Derethian day.
+///   </description></item>
+///   <item><description>
+///     Hour name (Darktide, Dawnsong, Midsong, Warmtide, Evensong,
+///     Gloaming and their "-and-Half" mid-hour variants).
+///   </description></item>
+///   <item><description>
+///     Day / night flag for spawn rules + lighting.
+///   </description></item>
+///   <item><description>
+///     Season / year / month for calendar UI.
+///   </description></item>
+/// </list>
+/// </para>
+///
+/// <para>
+/// Constants are retail-exact; changing them breaks the calendar panel's
+/// "today is" display.
+/// </para>
+/// </summary>
+public static class DerethDateTime
+{
+    public const int HoursInADay = 16;
+    public const int DaysInAMonth = 30;
+    public const int MonthsInAYear = 12;
+
+    /// <summary>Ticks per Derethian day (matches retail GameTime.DayLength).</summary>
+    public const double DayTicks = 7620.0;
+
+    /// <summary>Ticks per Derethian hour (DayTicks / 16).</summary>
+    public const double HourTicks = DayTicks / HoursInADay;     // 476.25
+
+    /// <summary>Ticks per Derethian month.</summary>
+    public const double MonthTicks = DayTicks * DaysInAMonth;    // 228,600
+
+    /// <summary>Ticks per Derethian year.</summary>
+    public const double YearTicks = MonthTicks * MonthsInAYear;  // 2,743,200
+
+    /// <summary>
+    /// Above this tick count the retail client crashes on connect.
+    /// ~1.07 billion seconds = PY 401 Thistledown 2 Morntide-and-Half.
+    /// </summary>
+    public const double MaxTicks = 1_073_741_828.0;
+
+    /// <summary>
+    /// The 16 named hour slots (r12 §1.2). Each one is half a Derethian
+    /// hour; the "-and-Half" variants are the second half.
+    /// </summary>
+    public enum HourName
+    {
+        Darktide = 0,
+        DarktideAndHalf,
+        Foredawn,
+        ForedawnAndHalf,
+        Dawnsong,           // day starts here (hour 5)
+        DawnsongAndHalf,
+        Morntide,
+        MorntideAndHalf,
+        Midsong,
+        MidsongAndHalf,
+        Warmtide,
+        WarmtideAndHalf,    // day ends here (hour 12)
+        Evensong,
+        EvensongAndHalf,
+        Gloaming,
+        GloamingAndHalf,
+    }
+
+    /// <summary>Derethian months (Snowreap..Frostfell, 12 total).</summary>
+    public enum MonthName
+    {
+        Snowreap = 0,
+        ColdMeet,
+        Leafdawning,
+        Seedsow,
+        Rosetide,
+        Solclaim,
+        Thistledown,
+        Harvestgain,
+        Leaftrue,
+        Reaptide,
+        Morningthaw,
+        Frostfell,
+    }
+
+    /// <summary>
+    /// Day fraction [0, 1): 0 = Darktide (midnight), 0.5 =
+    /// Midsong-and-Half (noon-ish), 1.0 wraps to 0.
+    /// </summary>
+    public static double DayFraction(double ticks)
+    {
+        if (ticks < 0) ticks = 0;
+        double rem = ticks - Math.Floor(ticks / DayTicks) * DayTicks;
+        return rem / DayTicks;
+    }
+
+    /// <summary>
+    /// Hour name for the given tick count. Picked from
+    /// <see cref="HourName"/> based on which named half-hour slot the
+    /// current time falls in.
+    /// </summary>
+    public static HourName CurrentHour(double ticks)
+    {
+        double f = DayFraction(ticks);
+        int slot = (int)Math.Floor(f * HoursInADay);
+        if (slot < 0) slot = 0;
+        if (slot > 15) slot = 15;
+        return (HourName)slot;
+    }
+
+    /// <summary>
+    /// True when the current time is "day" (Dawnsong through
+    /// Warmtide-and-Half, hours 5–12 inclusive on the 16-hour scale).
+    /// </summary>
+    public static bool IsDaytime(double ticks)
+    {
+        int h = (int)CurrentHour(ticks);
+        return h >= (int)HourName.Dawnsong && h <= (int)HourName.WarmtideAndHalf;
+    }
+
+    /// <summary>
+    /// Derethian calendar breakdown: (year, month, day, hour).
+    /// Year starts at PY 0. Day is 1-based within the month (1..30).
+    /// </summary>
+    public readonly record struct Calendar(int Year, MonthName Month, int Day, HourName Hour);
+
+    public static Calendar ToCalendar(double ticks)
+    {
+        if (ticks < 0) ticks = 0;
+        int year = (int)(ticks / YearTicks);
+        double tYear = ticks - year * YearTicks;
+        int monthIdx = (int)(tYear / MonthTicks);
+        if (monthIdx > 11) monthIdx = 11;
+        double tMonth = tYear - monthIdx * MonthTicks;
+        int day = (int)(tMonth / DayTicks) + 1;
+        if (day > DaysInAMonth) day = DaysInAMonth;
+
+        return new Calendar(year, (MonthName)monthIdx, day, CurrentHour(ticks));
+    }
+}