[WASM] `Perf_Utf8Encoding` and `XmlConfigurationProvider` benchmarks fail with `DirectoryNotFoundException`

WASM benchmarks that read test data files from disk fail with `DirectoryNotFoundException` because the files are not bundled into the WASM virtual filesystem. On WASM, `Environment.CurrentDirectory` returns `/`, so paths resolve to `/libraries/...` which don't exist in the VFS. **19 failures across all 11 partitions.** Affected tests got disabled in https://github.com/dotnet/performance/pull/5114.

### Error — `Perf_Utf8Encoding`

```
System.Reflection.TargetInvocationException: Arg_TargetInvocationException
---> System.IO.DirectoryNotFoundException: IO_PathNotFound_Path, /libraries/Common/EnglishAllAscii.txt
at System.IO.File.ReadAllText(String path)
at System.Text.Perf_Utf8Encoding.Setup()
at BenchmarkDotNet.Engines.Engine.Run()
// Benchmark Process 707781 has exited with code 255.
```

[full_log.txt](https://github.com/user-attachments/files/25600307/1.2.txt)

### Error — `XmlConfigurationProviderBenchmarks`

```
System.Reflection.TargetInvocationException: Arg_TargetInvocationException
---> System.IO.DirectoryNotFoundException: IO_PathNotFound_Path,
/libraries/Microsoft.Extensions.Configuration.Xml/TestFiles/simple.xml
at System.IO.File.OpenRead(String path)
at Microsoft.Extensions.Configuration.Xml.XmlConfigurationProviderBenchmarks.GlobalSetup()
at BenchmarkDotNet.Engines.Engine.Run()
// Benchmark Process 709659 has exited with code 255.
```

[full_log.txt](https://github.com/user-attachments/files/25600359/4.txt)

### Missing Files (9 total)

**`Perf_Utf8Encoding.Setup()` → `File.ReadAllText`:**
- `/libraries/Common/EnglishAllAscii.txt`
- `/libraries/Common/EnglishMostlyAscii.txt`
- `/libraries/Common/Chinese.txt`
- `/libraries/Common/Cyrillic.txt`
- `/libraries/Common/Greek.txt`

**`XmlConfigurationProviderBenchmarks.GlobalSetup()` → `File.OpenRead`:**
- `/libraries/Microsoft.Extensions.Configuration.Xml/TestFiles/simple.xml`
- `/libraries/Microsoft.Extensions.Configuration.Xml/TestFiles/deep.xml`
- `/libraries/Microsoft.Extensions.Configuration.Xml/TestFiles/names.xml`
- `/libraries/Microsoft.Extensions.Configuration.Xml/TestFiles/repeated.xml`

### Affected Benchmarks (19 failures)

- `System.Text.Perf_Utf8Encoding.{GetBytes,GetByteCount,GetString}` — 15 failures across inputs {EnglishAllAscii, EnglishMostlyAscii, Chinese, Cyrillic, Greek}
- `Microsoft.Extensions.Configuration.Xml.XmlConfigurationProviderBenchmarks.Load` — 4 failures across {simple.xml, deep.xml, names.xml, repeated.xml}

All 11 partitions are affected (1–4 failures each).

### Environment

```
BenchmarkDotNet v0.16.0-custom.20260225.100, Linux Ubuntu 22.04.5 LTS
AMD EPYC 9124 3.00GHz
Wasm : .NET Core (Mono) 11.0.0-ci, Wasm AOT VectorSize=128
Engine: v8-14.3.127 (--expose_wasm --module)
```

- **Job:** `Performance micro wasm wasm v8 linux x64 perfviper net11.0`
- **Job ID:** `925a1b8d-4fed-466f-9da4-8815e3953218`

### Root Cause

The test data files are not bundled into the WASM publish output. The benchmark `[GlobalSetup]` methods use `Path.Combine(Environment.CurrentDirectory, "libraries", ...)` which resolves to `/libraries/...` on WASM (where CWD is `/`), and those paths don't exist in the VFS.

### Fix Options

1. Use `AppContext.BaseDirectory` instead of `Environment.CurrentDirectory` for path resolution
2. Bundle the test data files into the WASM virtual filesystem during build
3. Embed the test data as resources and extract at runtime