Two CODESTYLE conformance gaps found in the fleet audit (rules added in ptr727/ProjectTemplate#270).

1. Tests: xUnit v3 + AwesomeAssertions (CODESTYLE §Testing Conventions)

Current: Directory.Packages.props pins xunit 2.9.3 (legacy v2), and UtilitiesTests/*.cs use using Xunit + native asserts (Assert.Equal, Assert.True, ...).
Target:

• Replace the v2 xunit package with xunit.v3 (3.2.2, matching the fleet).
• Add AwesomeAssertions and convert every Assert.* to the fluent .Should() API.
• Reference peers already on this: LanguageTags, PlexCleaner, PhotoCleaner, MediaTools, NxWitness.

2. Library logging via ILoggerFactory, drop Serilog (CODESTYLE §Error Handling and Logging)

Current: Utilities/Utilities.csproj references Serilog directly, forcing a logging framework on every consumer (and it is not AOT-annotated).
Target:

• The library depends only on Microsoft.Extensions.Logging.Abstractions and exposes an ILoggerFactory seam (settable factory defaulting to NullLoggerFactory, SetFactory/TrySetFactory).
• Any consuming app owns the concrete Serilog logger and injects it via SerilogLoggerFactory.
• Reference implementation: LanguageTags — LanguageTags/LogOptions.cs (library seam) + LanguageTagsCreate/LoggerFactory.cs (app-side Serilog bridge).
• Note: this changes the library's public logging surface (semver-major).

Audited from develop, 2026-07-09.