feat: Adds Latin1 text support (#2317)

## Summary

- Adds proper Latin1 encoding support, replacing CP1252 usage throughout the codebase
- Adds specialized, optimized string decoding methods with safe string filtering for each encoding type
- Filters invalid Unicode characters (C0/C1 control codes, non-characters) by removal rather than replacement since
the UO client renders nothing for these characters
- Fixes UTF-16 null terminator position handling to correctly advance by 2 bytes

## Changes

TextEncoding.cs

- Added SearchValues-based invalid byte/char detection for efficient filtering
- Added encoding-specific GetString methods: GetStringAscii, GetStringLatin1, GetStringUtf8, GetStringBigUni,
GetStringLittleUni
- Each method supports a safeString parameter for filtering invalid characters
- Little-endian UTF-16 uses direct memory cast for zero-copy decoding on LE systems
- Invalid characters are removed (not replaced with U+FFFD) since the client renders nothing for them

SpanReader.cs

- Added ReadLatin1() and ReadLatin1Safe() methods
- Rewrote encoding-specific read methods to use optimized TextEncoding.GetString* methods
- Fixed UTF-16 null terminator handling: position now correctly advances by byteLength (2) instead of 1

SpanWriter.cs

- Added WriteLatin1 and WriteLatin1Null methods

## Packet Updates

- Updated all packet code to use Latin1 encoding instead of CP1252
- Affected: account packets, equipment packets, menu packets, message packets, mobile packets, player packets, secure
trade packets, vendor packets, gump packets, book packets, mahjong packets

## Filtering Behavior

Invalid characters filtered in safe mode:
```
┌───────────────┬────────────────────────┐
│     Range     │      Description       │
├───────────────┼────────────────────────┤
│ 0x00-0x1F     │ C0 control codes       │
├───────────────┼────────────────────────┤
│ 0x7F          │ DEL                    │
├───────────────┼────────────────────────┤
│ 0x80-0x9F     │ C1 control codes       │
├───────────────┼────────────────────────┤
│ 0xFFFE-0xFFFF │ Unicode non-characters │
└───────────────┴────────────────────────┘
```

Note: Surrogate pairs (0xD800-0xDFFF) are not filtered because proper validation requires context checking for paired
vs unpaired surrogates. The UO client renders nothing for these anyway.

## Test Plan

- All 631 Server.Tests pass
- Verified client rendering behavior using TestUnicodeGump command (pages 1-5)
- Confirmed U+FFFD, unpaired surrogates, and non-characters all render as blank in client
- Verified Latin1 characters (0xA0-0xFF) display correctly
- Verified C1 control codes (0x80-0x9F) are filtered and don't display

Projects/Server.Tests/Tests/Buffers/SpanReaderTests.cs

@@ -291,7 +291,8 @@ public class SpanReaderTests
         var result = reader.ReadLittleUni();
 
         Assert.Equal("Hi", result);
-        Assert.Equal(5, reader.Position);
+        // Position 6: 4 bytes for "Hi" + 2 bytes for UTF-16 null terminator
+        Assert.Equal(6, reader.Position);
     }
 
     [Fact]
@@ -309,12 +310,14 @@ public class SpanReaderTests
     [Fact]
     public void TestReadLittleUniSafe()
     {
-        ReadOnlySpan<byte> buffer = [(byte)'H', 0, 0xFF, 0xD8, (byte)'i', 0];
+        // Test with C1 control code (0x85 = NEL) which should be filtered
+        ReadOnlySpan<byte> buffer = [(byte)'H', 0, 0x85, 0x00, (byte)'i', 0];
         var reader = new SpanReader(buffer);
 
         var result = reader.ReadLittleUniSafe();
 
-        Assert.Equal("H\uFFFDi", result);
+        // C1 control (0x0085) removed - client renders nothing for invalid chars
+        Assert.Equal("Hi", result);
         Assert.Equal(6, reader.Position);
     }
 
@@ -339,7 +342,8 @@ public class SpanReaderTests
         var result = reader.ReadBigUni();
 
         Assert.Equal("Hi", result);
-        Assert.Equal(5, reader.Position);
+        // Position 6: 4 bytes for "Hi" + 2 bytes for UTF-16 null terminator
+        Assert.Equal(6, reader.Position);
     }
 
     [Fact]
@@ -357,12 +361,14 @@ public class SpanReaderTests
     [Fact]
     public void TestReadBigUniSafe()
     {
-        ReadOnlySpan<byte> buffer = [0, (byte)'H', 0xD8, 0xFF, 0, (byte)'i'];
+        // Test with C1 control code (0x0085 = NEL) which should be filtered
+        ReadOnlySpan<byte> buffer = [0, (byte)'H', 0x00, 0x85, 0, (byte)'i'];
         var reader = new SpanReader(buffer);
 
         var result = reader.ReadBigUniSafe();
 
-        Assert.Equal("H\uFFFDi", result);
+        // C1 control (0x0085) removed - client renders nothing for invalid chars
+        Assert.Equal("Hi", result);
         Assert.Equal(6, reader.Position);
     }