perf: keep damage entries in an inline intrusive list (#2605)

## Summary

`Mobile.DamageEntries` was a `List<DamageEntry>` allocated for every mobile, including the ~99% that never take damage. It is now an inline `ValueLinkList<DamageEntry>` (24 bytes in the `Mobile` object, no separate allocation) ordered least recent → most recent.

- `DamageEntry` implements `IValueLinkListNode<DamageEntry>`.
- `RegisterDamage` moves the entry to the tail in O(1) instead of `Remove` + `Add` on a list.
- Expired entries are always a head prefix, so pruning walks from the head and stops at the first live entry. The `DamageEntries` getter prunes on access.
- `DamageEntries` is exposed as `ref readonly`; enumerate with `foreach` or `.ByDescending()`. Mutation goes through `RegisterDamage` / `ClearDamageEntries`.
- `BaseCreature.GetLootingRights` and `BaseCreature.ComputeBonusDamage` take `in ValueLinkList<DamageEntry>`; all callers compile unchanged. Files that `foreach` over `DamageEntries` need `using Server.Collections;` for the enumerator extension.
- RunUO migration docs (`dev-docs/runuo-migration-docs/09`, `11`) and the `migrate-items-mobiles` skill document the change.

Saves one object and 16 bytes per mobile (~8 MB and 500k gen2 objects on a 500k world). Second of three PRs from the lazy per-mobile collections design (first: #2604). Branched from `main`; the two diffs touch disjoint hunks of `Mobile.cs`.

## Breaking change

- `Mobile.DamageEntries` is no longer a `List<DamageEntry>`. Indexing, `.Clear()`, `.Add()`, `.Remove()` no longer compile; use `foreach`, `.ByDescending()`, `.Count`, `ClearDamageEntries()`, and `RegisterDamage`. Calling a `ValueLinkList` mutator on the `ref readonly` property compiles but operates on a copy while still unlinking the real nodes; do not.
- `BaseCreature.GetLootingRights` and `BaseCreature.ComputeBonusDamage` signatures changed to `(in ValueLinkList<DamageEntry>, …)`.

Save format is untouched: damage entries are not serialized.

## Behavior

Recency order, `allowSelf`, tie-breaking in `FindMostTotal`/`FindLeastTotal` (most recent wins), `Responsible` accounting, and loot-rights ordering are unchanged and covered by the new `DamageEntryTests` and `LootingRightsTests`.

## Testing

- `dotnet build -c Release` clean.
- New `DamageEntryTests` and `LootingRightsTests` plus full `Server.Tests` and `UOContent.Tests`.

Projects/UOContent/Engines/CannedEvil/ChampionSpawn.cs

@@ -18,6 +18,7 @@ using System.Net;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using ModernUO.Serialization;
+using Server.Collections;
 using Server.Engines.Virtues;
 using Server.Gumps;
 using Server.Items;
@@ -1181,11 +1182,6 @@ public partial class ChampionSpawn : Item
 
         foreach (var de in m.DamageEntries)
         {
-            if (de.HasExpired)
-            {
-                continue;
-            }
-
             var damager = de.Damager;
             var master = damager.GetDamageMaster(m);
 

Projects/UOContent/Mobiles/BaseCreature.cs

@@ -3123,14 +3123,12 @@ namespace Server.Mobiles
             return base.OnBeforeDeath();
         }
 
-        public int ComputeBonusDamage(List<DamageEntry> list, Mobile m)
+        public int ComputeBonusDamage(in ValueLinkList<DamageEntry> list, Mobile m)
         {
             var bonus = 0;
 
-            for (var i = list.Count - 1; i >= 0; --i)
+            foreach (var de in list.ByDescending())
             {
-                var de = list[i];
-
                 if (de.Damager == m || de.Damager is not BaseCreature bc)
                 {
                     continue;
@@ -3167,26 +3165,15 @@ namespace Server.Mobiles
             Combatant is PlayerMobile ||
             Combatant is BaseCreature { Controlled: true } bc && bc.GetMaster() is PlayerMobile;
 
-        public static List<DamageStore> GetLootingRights(List<DamageEntry> damageEntries, int hitsMax)
+        // Iterates most recent first, matching the previous reverse-indexed loop. The list is
+        // already pruned of expired entries by the Mobile.DamageEntries getter.
+        public static List<DamageStore> GetLootingRights(in ValueLinkList<DamageEntry> damageEntries, int hitsMax)
         {
             var rights = new List<DamageStore>();
             DamageStore firstDamager = null;
 
-            for (var i = damageEntries.Count - 1; i >= 0; --i)
+            foreach (var de in damageEntries.ByDescending())
             {
-                if (i >= damageEntries.Count)
-                {
-                    continue;
-                }
-
-                var de = damageEntries[i];
-
-                if (de.HasExpired)
-                {
-                    damageEntries.RemoveAt(i);
-                    continue;
-                }
-
                 var damage = de.DamageGiven;
 
                 var respList = de.Responsible;

Projects/UOContent/Mobiles/Special/Harrower.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using ModernUO.Serialization;
+using Server.Collections;
 using Server.Engines.CannedEvil;
 using Server.Engines.Virtues;
 using Server.Items;