sky(phase-3a): per-Dereth-day weather roll + ACDREAM_DAY_GROUP override

Diagnosed the "retail shows early night, acdream shows early day" time
mismatch: server time sync was working correctly (ticks=291079558 →
dayFraction=0.8546 → EvensongAndHalf, hour 14 of 16). The mismatch was
the hardcoded DayGroup index.

Dereth's SkyDesc carries 20 DayGroups (Sunny / Clear / Cloudy / Storm /
etc), each weighted at ChanceOfOccur=5.0. Retail rolls one per server
day by `ChanceOfOccur` as a PDF (r12 §11). We were always rendering
DayGroup 0 = "Sunny" regardless of day, so at EvensongAndHalf we showed
SkyTime[7]@0.84 — sun still 20° above the western horizon, warm golden
— i.e. pre-sunset rather than the dimmer pre-night appearance retail
shows after rolling a cloudier group.

Fix (Phase 3a):
- LoadedSkyDesc.SelectDayGroupIndex(dayIndex) — deterministic roller:
  SplitMix64 hash of dayIndex → normalize to [0, sumChances) → walk the
  cumulative distribution. Same dayIndex on every client = same weather
  on every client, zero network sync needed.
- LoadedSkyDesc.ActiveDayGroup(ticks) / BuildProviderForDay(ticks) —
  convenience wrappers that compute dayIndex from raw server ticks.
- ACDREAM_DAY_GROUP=<N> env var override. Set to 10 "Clear", 12 "Cloudy",
  etc. for A/B visual verification against retail.
- SyncFromServer gains a [sky-dump] log: `ticks=X dayFraction=Y
  calendar=PY{year} {month} {day} {hour}` so the time-sync state is
  auditable from a single grep.
- GameWindow: tracks _loadedSkyDayIndex + _activeDayGroup. Calls
  RefreshSkyForCurrentDay on every server sync — swaps WorldTime's
  provider + caches the group only when the day index crosses a
  boundary (idempotent within a single day). SkyRenderer.Render now
  consumes _activeDayGroup instead of the legacy DefaultDayGroup.

Observed (this session, local ACE):
  server sent ticks=291079558 → PY106 ColdMeet 10 EvensongAndHalf
  SplitMix64(day 38197) will deterministically pick one of 20 groups.

Build + 717 tests green. Ready for user visual verification with
retail side-by-side.

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

src/AcDream.Core/World/SkyState.cs

@@ -289,8 +289,17 @@ public sealed class WorldTimeService
     public void SyncFromServer(double serverTicks)
     {
         _lastSyncedTicks = serverTicks;
-        _lastSyncedWallClockUtc = DateTime.UtcNow;
+        _lastSyncedWallClockUtc = System.DateTime.UtcNow;
         _debugDayFractionOverride = null;
+
+        if (System.Environment.GetEnvironmentVariable("ACDREAM_DUMP_SKY") == "1")
+        {
+            var df = DerethDateTime.DayFraction(serverTicks);
+            var cal = DerethDateTime.ToCalendar(serverTicks);
+            System.Console.WriteLine(
+                $"[sky-dump] SyncFromServer: ticks={serverTicks:F1} dayFraction={df:F4} " +
+                $"calendar=PY{cal.Year} {cal.Month} {cal.Day} {cal.Hour}");
+        }
     }
 
     /// <summary>