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`.

dev-docs/runuo-migration-docs/09-items-mobiles-creatures.md

@@ -517,6 +517,45 @@ periodic `ReacquireDelay` scan still sweeps the full `RangePerception`). The
 acquired target comes from the normal FightMode-ranked scan, not from whichever mobile
 happened to move. See `content-patterns.md` § Target Acquisition.
 
+## Damage Entries: Inline `ValueLinkList`, Not `List<DamageEntry>`
+
+RunUO's `Mobile.DamageEntries` was a `List<DamageEntry>` allocated for every mobile.
+ModernUO keeps damage entries in an inline `ValueLinkList<DamageEntry>` struct held by
+the mobile itself, ordered least recent → most recent, so a mobile that never takes
+damage owns no list object and `RegisterDamage` relinks in O(1). The property is
+`ref readonly`; expired entries are pruned when it is read.
+
+```csharp
+// RunUO
+for (var i = m.DamageEntries.Count - 1; i >= 0; --i)
+{
+    var de = m.DamageEntries[i];          // indexer
+    ...
+}
+m.DamageEntries.Clear();
+var rights = BaseCreature.GetLootingRights(m.DamageEntries, m.HitsMax); // List<DamageEntry>
+
+// ModernUO — needs `using Server.Collections;` for the enumerator extensions
+foreach (var de in m.DamageEntries.ByDescending()) // most recent first
+{
+    ...
+}
+foreach (var de in m.DamageEntries)                // least recent first
+{
+    ...
+}
+m.ClearDamageEntries();
+var rights = BaseCreature.GetLootingRights(m.DamageEntries, m.HitsMax); // in ValueLinkList<DamageEntry>
+```
+
+What no longer compiles: the indexer, `.Add`, `.Remove`, `.RemoveAt`, `.Clear`, and
+passing the property where a `List<DamageEntry>` is expected. `.Count`,
+`FindDamageEntryFor`, `FindMostRecentDamager` and the other `Find*` methods, and
+`RegisterDamage` are unchanged. `DamageEntry` now carries `Next`/`Previous`/`OnLinkList`
+link fields; never set them yourself, and never call a `ValueLinkList` mutator on the
+`ref readonly` property — it compiles against a copy and corrupts the node's link state.
+Mutate only through `RegisterDamage` and `ClearDamageEntries`.
+
 ## Item Name Changes
 
 ```csharp

dev-docs/runuo-migration-docs/11-api-reference.md

@@ -134,6 +134,9 @@ Alphabetical by RunUO API name. Use Ctrl+F / Cmd+F to search.
 | `WalkMobileRange(m, steps, run, min, max)` | `WalkMobileRange(m, steps, min, max)` | Same |
 | `PathFollower.Follow(run, range)` | `Follow(range)` | Same |
 | `AcquireOnApproach` (bool) | `AcquireOnApproachDelay` (TimeSpan) | Reaction-time gradient; `Zero` = old instant behavior |
+| `m.DamageEntries` (`List<DamageEntry>`) | `m.DamageEntries` (`ref readonly ValueLinkList<DamageEntry>`) | Inline, least→most recent; `foreach` / `.ByDescending()` only, needs `using Server.Collections;`; no indexer, `Add`, `Remove`, `Clear` |
+| `m.DamageEntries.Clear()` | `m.ClearDamageEntries()` | |
+| `GetLootingRights(List<DamageEntry>, int)` | `GetLootingRights(in ValueLinkList<DamageEntry>, int)` | Callers passing `m.DamageEntries` compile unchanged |
 
 ## Networking