fix(time): retail-canonical month enum + absolute Portal Year + title-bar calendar

Two bugs in calendar display (the CLOCK ITSELF was already correct):

1. **Month enum had wrong order + non-retail names.** Old enum:
   Snowreap=0, ColdMeet, Leafdawning, Seedsow, Rosetide, Solclaim, ...
   At day-of-year 83 this gave month index 2 = Leafdawning. Retail's
   @timestamp at the same moment shows "Seedsow 24". Fixed enum to
   chronological order starting at year-anchor month Morningthaw, with
   retail-canonical names:
     Morningthaw=0, Solclaim, Seedsow, Leafdawning, Verdantine,
     Thistledown, Harvestgain, Leafcull, Frostfell, Snowreap,
     Coldeve, Wintersebb.
   At day-of-year 83 → month 2 = Seedsow ✓

2. **ToCalendar returned relative year, not absolute Portal Year.**
   We had AbsoluteYear() = relative_year + ZeroYear (=10) but
   ToCalendar's Calendar.Year was the relative one. So acdream's
   title bar showed "PY 106" while retail's @timestamp at the same
   tick showed "PY 116". Fixed ToCalendar to add ZeroYear so the
   exposed Calendar.Year matches retail's display.

3. **GameWindow title bar now shows the calendar.** Format mirrors
   retail's @timestamp output:
     "PY<Year> <Month> <Day> <Hour> (df=<dayFraction>)"
   Lets the user read the same fields off both clients and confirm
   clock parity directly. Drift > 1 hour = real bug.

Tests:
- Updated ToCalendar_PY10Day1_Morningthaw (renamed from PY0Day1_Snowreap)
- Updated ToCalendar_AdvancesCorrectly (Snowreap→Morningthaw etc.)
- Added regression: ToCalendar_TickAtSeedsow24Year106_MatchesRetailFormat
  pinning a retail-known tick → retail-known calendar string.

The dayFraction formula (CalcDayBegin's `arg2 + zero_time_of_year`,
decomp 0x005a6400 line 434549) was already correct; an earlier-this-
session attempt to flip the sign was reverted in this same commit's
parent. The "few minutes drift" observed in dual-client comparisons
this session was a combination of:
  - calendar label mismatch (this fix addresses)
  - slot-boundary rounding (fixes itself)
  - 1-minute wall-clock interpolation drift (within tolerance)

NOT a clock-formula bug. ISSUE #3 in docs/ISSUES.md is now misnamed
("Client clock drifts from retail"); plan to re-title or close in a
follow-up commit after the visual-divergence investigation lands.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -4565,9 +4565,18 @@ public sealed class GameWindow : IDisposable
             int entityCount = _worldState.Entities.Count;
             int animatedCount = _animatedEntities.Count;
 
-            _window!.Title = $"acdream | {fps:F0} fps | {avgFrameTime:F1} ms | " +
+            // Calendar display matches retail's @timestamp output:
-                             $"lb {visibleLandblocks}/{totalLandblocks} visible | " +
+            // "Date: <Month> <Day>, PY <Year>  Time: <HourName>".
-                             $"ent {entityCount} | anim {animatedCount}";
+            // Use NowTicks (server-synced + wall-clock interpolation) so the
+            // user can read the same fields off both acdream and retail and
+            // confirm clock parity directly. Drift > 1 hour = real bug.
+            double tNow = WorldTime.NowTicks;
+            var titleCal = AcDream.Core.World.DerethDateTime.ToCalendar(tNow);
+            double df = WorldTime.DayFraction;
+            _window!.Title =
+                $"acdream | {fps:F0} fps | {avgFrameTime:F1} ms | " +
+                $"lb {visibleLandblocks}/{totalLandblocks} | ent {entityCount}/anim {animatedCount} | " +
+                $"PY{titleCal.Year} {titleCal.Month} {titleCal.Day} {titleCal.Hour} (df={df:F4})";
             _lastFps = fps;
             _lastFrameMs = avgFrameTime;
             _perfAccum = 0;

src/AcDream.Core/World/DerethDateTime.cs

@@ -90,21 +90,30 @@ public static class DerethDateTime
         GloamingAndHalf,
     }
 
-    /// <summary>Derethian months (Snowreap..Frostfell, 12 total).</summary>
+    /// <summary>
+    /// Derethian months in chronological order. Year-0 begins at month 0
+    /// (<see cref="Morningthaw"/>) and progresses through the 12-month
+    /// cycle. Names + order match retail's calendar display
+    /// (<c>GameTime::CalcDayBegin</c> + <c>GetDateTimeString</c> at
+    /// <c>0x005a6530</c>) and ACE's <c>DerethDateTime.cs</c>. Verified
+    /// against retail's <c>@timestamp</c> output in 2026-04-27 dual-
+    /// client comparison: at day-of-year 83, retail shows
+    /// "Seedsow 24" — that fixes month index 2 = Seedsow.
+    /// </summary>
     public enum MonthName
     {
-        Snowreap = 0,
+        Morningthaw = 0,
-        ColdMeet,
-        Leafdawning,
-        Seedsow,
-        Rosetide,
         Solclaim,
+        Seedsow,
+        Leafdawning,
+        Verdantine,
         Thistledown,
         Harvestgain,
-        Leaftrue,
+        Leafcull,
-        Reaptide,
-        Morningthaw,
         Frostfell,
+        Snowreap,
+        Coldeve,
+        Wintersebb,
     }
 
     /// <summary>
@@ -127,12 +136,15 @@ public static class DerethDateTime
     /// for the boot window before the dat parses.
     ///
     /// <para>
-    /// Live Dereth dat value: <c>3600</c>. The +7/16 default is wrong
+    /// Live Dereth dat value: <c>3600</c>. Retail's
-    /// by 266.25 ticks (~33 Derethian minutes) and was the source of
+    /// <c>GameTime::CalcDayBegin</c> at <c>0x005a6400</c> (decomp line
-    /// the "acdream time is behind retail" + "wrong DayGroup picked"
+    /// 434549) computes <c>arg2 + zero_time_of_year</c> as the basis for
-    /// observations in the 2026-04-23 live verification session — see
+    /// year/day-of-year extraction, then derives <c>time_of_day_begin</c>
-    /// <c>docs/research/2026-04-23-daygroup-selection.md</c> §4 and
+    /// such that <c>(arg2 - time_of_day_begin) / day_length</c> in
-    /// the Phase 3f commit.
+    /// <c>CalcTimeOfDay</c> gives <c>(arg2 + zero_time_of_year) mod day_length / day_length</c>.
+    /// Net: the formula is ADD, not subtract — confirmed via the explicit
+    /// add at line 434549. (A 2026-04-26 attempt to flip the sign over-
+    /// corrected and broke DG selection; reverted in the same commit.)
     /// </para>
     /// </summary>
     public static double OriginOffsetTicks { get; private set; } = DayFractionOriginOffsetTicks;
@@ -186,7 +198,10 @@ public static class DerethDateTime
 
     /// <summary>
     /// Derethian calendar breakdown: (year, month, day, hour).
-    /// Year starts at PY 0. Day is 1-based within the month (1..30).
+    /// <see cref="Year"/> is the absolute Portal Year (= relative-year +
+    /// <see cref="ZeroYear"/>) so the value matches retail's
+    /// <c>@timestamp</c> output ("Date: &lt;Month&gt; &lt;Day&gt;,
+    /// &lt;Year&gt; P.Y."). Day is 1-based within the month (1..30).
     /// </summary>
     public readonly record struct Calendar(int Year, MonthName Month, int Day, HourName Hour);
 
@@ -194,15 +209,19 @@ public static class DerethDateTime
     {
         if (ticks < 0) ticks = 0;
         double shifted = ticks + OriginOffsetTicks;
-        int year = (int)(shifted / YearTicks);
+        int relativeYear = (int)(shifted / YearTicks);
-        double tYear = shifted - year * YearTicks;
+        double tYear = shifted - relativeYear * 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));
+        // Absolute Portal Year for display: retail's @timestamp shows
+        // PY-with-base (10 P.Y. == year 0 of the calendar epoch), so add
+        // ZeroYear here. Matches AbsoluteYear() and the retail decomp at
+        // FUN_005a7510:5300.
+        return new Calendar(relativeYear + ZeroYear, (MonthName)monthIdx, day, CurrentHour(ticks));
     }
 
     /// <summary>

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

@@ -75,26 +75,56 @@ public sealed class DerethDateTimeTests
     }
 
     [Fact]
-    public void ToCalendar_PY0Day1_Snowreap()
+    public void ToCalendar_PY10Day1_Morningthaw()
     {
+        // Tick 0 maps to PY 10 (= relative year 0 + ZeroYear=10),
+        // Morningthaw 1 — matches retail's calendar epoch
+        // (ACE DerethDateTime.cs: dayZeroTicks = 0; // Morningthaw 1, 10 P.Y.).
         var cal = DerethDateTime.ToCalendar(0);
-        Assert.Equal(0, cal.Year);
+        Assert.Equal(DerethDateTime.ZeroYear, cal.Year);
-        Assert.Equal(DerethDateTime.MonthName.Snowreap, cal.Month);
+        Assert.Equal(DerethDateTime.MonthName.Morningthaw, cal.Month);
         Assert.Equal(1, cal.Day);
     }
 
     [Fact]
     public void ToCalendar_AdvancesCorrectly()
     {
-        // One year from start → PY 1, Snowreap 1.
+        // One year from start → PY (10 + 1) = 11, Morningthaw 1.
         var cal = DerethDateTime.ToCalendar(DerethDateTime.YearTicks);
-        Assert.Equal(1, cal.Year);
+        Assert.Equal(DerethDateTime.ZeroYear + 1, cal.Year);
-        Assert.Equal(DerethDateTime.MonthName.Snowreap, cal.Month);
+        Assert.Equal(DerethDateTime.MonthName.Morningthaw, cal.Month);
         Assert.Equal(1, cal.Day);
 
-        // One month into year 1.
+        // One month into year 11 → Solclaim (next month after Morningthaw).
         var cal2 = DerethDateTime.ToCalendar(DerethDateTime.YearTicks + DerethDateTime.MonthTicks);
-        Assert.Equal(1, cal2.Year);
+        Assert.Equal(DerethDateTime.ZeroYear + 1, cal2.Year);
-        Assert.Equal(DerethDateTime.MonthName.ColdMeet, cal2.Month);
+        Assert.Equal(DerethDateTime.MonthName.Solclaim, cal2.Month);
+    }
+
+    [Fact]
+    public void ToCalendar_TickAtSeedsow24Year106_MatchesRetailFormat()
+    {
+        // Regression guard for the 2026-04-27 dual-client comparison.
+        // Retail @timestamp output format is
+        //   "Date: <Month> <Day>, <Year> P.Y."
+        // Pick a tick at the exact start of Seedsow 24 in relative year 106:
+        //   shifted = 106 * YearTicks + 2 * MonthTicks + 23 * DayTicks
+        // Derived: 290,779,200 + 457,200 + 175,260 = 291,411,660. Subtract
+        // OriginOffsetTicks (3600 in Dereth dat) to get the input tick:
+        //   291,411,660 - 3600 = 291,408,060
+        // Expected output: PY 116 (= ZeroYear 10 + relative 106), Seedsow,
+        // day 24 1-indexed.
+        DerethDateTime.SetOriginOffsetFromDat(3600.0);
+        try
+        {
+            var cal = DerethDateTime.ToCalendar(291_408_060.0);
+            Assert.Equal(DerethDateTime.ZeroYear + 106, cal.Year);
+            Assert.Equal(DerethDateTime.MonthName.Seedsow, cal.Month);
+            Assert.Equal(24, cal.Day);
+        }
+        finally
+        {
+            DerethDateTime.SetOriginOffsetFromDat(DerethDateTime.DayFractionOriginOffsetTicks);
+        }
     }
 }