feat: anchored-time infrastructure with a save-start anchor in idx v5 (#2585)

## Summary

The save-stability infrastructure consumed by generator v3's `[AnchoredDateTime]`: anchored timestamps are written as **absolute values** and re-based once at load by the elapsed time since the save started — so downtime doesn't age them, and an unchanged entity serializes to identical bytes (the prerequisite for replacing delta-time encodings, which rewrite every entity on every save).

## Design

- **`WriteAnchoredTime` / `ReadAnchoredTime`** on `IGenericWriter`/`IGenericReader`. The read side applies the reader's `AnchoredTimeShift`; `Min/MaxValue` sentinels pass through unshifted, and shifts saturate instead of overflowing.
- **`World.SaveStartTime`** is stamped the moment the world freezes for a snapshot — one anchor for the entire save, no per-persistence skew.
- **idx v5**: the anchor ticks sit in the header right after the version. The anchor travels with the file it re-anchors, so a single idx+bin pair restored from a backup is self-describing, and anchor presence is guaranteed by the same version gate as the record format — there is no separate anchor file to lose.
- **The shift rides the reader instance** (`BufferReader`, `UnmanagedDataReader`, `BinaryFileReader` delegating), not a static — parallel per-persistence loads and ad-hoc restores each see their own file's anchor. idx v4 and older read with a zero shift.

## Scope

Behavior-neutral: nothing serializes anchored values yet (`Item.DecayResetTime` and the `[DeltaDateTime]` field migrations come separately, with their own version bumps). Saves written from this branch are idx v5; loading v4/v3 saves is unchanged and remains pinned by the existing hand-written-header tests.

## Testing

- Unit round-trips: exact with zero shift, shifted read, sentinel passthrough, saturation, Local→UTC normalization.
- End-to-end through the real worker/segment-log pipeline: an anchored timestamp re-bases across a simulated two-hour downtime via the idx v5 header.
- Full suites green: Server.Tests 835/835, UOContent.Tests 708/708 (including the existing v4/v3 idx loading tests).

Projects/Server/World/World.cs

@@ -93,6 +93,12 @@ public static class World
     public static string SavePath { get; private set; }
     public static WorldState WorldState { get; private set; }
     public static bool Saving => WorldState == WorldState.Saving;
+
+    /// <summary>
+    /// UTC time the current or most recent world save started. Written into save indexes so
+    /// anchored timestamps can be re-based by the downtime at load.
+    /// </summary>
+    public static DateTime SaveStartTime { get; internal set; }
     public static bool Running => WorldState is not WorldState.Loading and not WorldState.Initial;
     public static bool Loading => WorldState == WorldState.Loading;
 
@@ -287,6 +293,10 @@ public static class World
 
         WorldState = WorldState.Saving;
 
+        // The world is frozen from here: one anchor for the whole save. Written into save
+        // indexes so anchored timestamps can be re-based by the downtime at load.
+        SaveStartTime = Core.Now;
+
         Broadcast(0x35, true, "The world is saving, please wait.");
 
         logger.Information("Saving world");