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/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