perf: Zero-alloc interpolation for SendMessage/Overhead APIs (#2434)

## Summary

Phase 1 of a multi-phase optimization to eliminate intermediate string allocations between `$"..."` interpolation and the packet text region for ModernUO's player-facing message APIs.

- Adds `[InterpolatedStringHandler]` overloads to every `Send*`/`Public/Local/Private/NonlocalOverheadMessage`/`Say`/`Emote`/`Whisper`/`Yell`/`SendLocalizedMessageTo` API in `OutgoingMessagePackets`, `Mobile`, and `Item`. Each overload is a 3-line shim that forwards `handler.Text` to the existing span-based path then calls `handler.Clear()` to return the rented `STArrayPool<char>` buffer (matches the established `SpanWriter.WriteAscii(ref RawInterpolatedStringHandler)` precedent).
- Converts `string text/args/affix/name` parameters to `ReadOnlySpan<char>` for consistency with the handler path. `lang` intentionally stays `string` (it's never interpolated and the `??= "ENU"` fallback stays cleaner).
- Adds `int charCount` overloads of the three `GetMaxMessage*Length` helpers so stackalloc sizing can avoid the redundant `ROS<char>` round-trip.
- Moves `Mobile` (17 methods) and `Item` (4 methods) message methods into new partial-class files (`Mobile.Messages.cs`, `Item.Messages.cs`) for organization.

No UOContent call sites change in this PR — existing `string`/`ROS<char>` calls compile unchanged via implicit conversion. Phase 2 (intermediate-string audit) and Phase 3 (cleanup PRs) follow.

## Files

- `Projects/Server/Network/Packets/OutgoingMessagePackets.cs` — `string` → `ROS<char>` for text params, `int charCount` length helpers added, class made `partial`
- `Projects/Server/Network/Packets/OutgoingMessagePackets.Interpolated.cs` (new) — 3 `ref RawInterpolatedStringHandler` extension overloads
- `Projects/Server/Mobiles/Mobile.cs` — message methods extracted (-262 lines)
- `Projects/Server/Mobiles/Mobile.Messages.cs` (new, 463 lines) — moved + ROS-converted methods + 25 handler overloads
- `Projects/Server/Items/Item.cs` — message methods extracted (-93 lines)
- `Projects/Server/Items/Item.Messages.cs` (new, 142 lines) — moved + ROS-converted methods + 4 handler overloads
- `Projects/Server.Tests/Tests/Network/Packets/Outgoing/MessagePacketTests.cs` — 3 new regression tests verifying byte-equivalence for the handler overloads

Projects/Server/World/World.cs

@@ -110,7 +110,7 @@ public static class World
 
     public static void Broadcast(int hue, bool ascii, string text)
     {
-        var length = OutgoingMessagePackets.GetMaxMessageLength(text);
+        var length = OutgoingMessagePackets.GetMaxMessageLength(text.Length);
 
         var buffer = stackalloc byte[length].InitializePacket();
 
@@ -140,7 +140,7 @@ public static class World
 
     public static void BroadcastStaff(int hue, bool ascii, string text)
     {
-        var length = OutgoingMessagePackets.GetMaxMessageLength(text);
+        var length = OutgoingMessagePackets.GetMaxMessageLength(text.Length);
 
         var buffer = stackalloc byte[length].InitializePacket();