fix: make benchmark host builds resolve the generator correctly

[kieronlanning]

Fixes the benchmark suite so BenchmarkDotNet's autogenerated host build can compile and run.

Root cause: the host build passes \ArtifactsPath/\OutDir/\OutputPath\ and \UseSharedCompilation=false\ as global properties. Those propagated into the generator project (via its analyzer ProjectReference) and relocated its build output into stray folders relative to the project, and Roslyn's in-process analyzer loader then could not resolve the generator's \Purview.SourceGeneratorFramework\ runtime dependencies.

Changes:

• \src/Directory.Build.props: keep intermediates in \obj/\ via \UseArtifactsOutput=false\ even when a global \ArtifactsPath\ is present.
• \�enchmarks/Directory.Build.props\ (new): same for the benchmark project.
• \Purview.Telemetry.Benchmarks.csproj: strip \ArtifactsPath;OutDir;OutputPath\ from the generator ProjectReference globals, and load the two \Purview.SourceGeneratorFramework\ runtime assemblies as analyzers so they are present in the analyzer load context under \UseSharedCompilation=false.

Verified: filtered Release benchmark run on net10.0 executes end-to-end (32 benchmarks, no host-build failure); main + sample solutions build; csharpier lint passes.