Optimize export lookup and simple activation - #772

Summary

• cache exact runtime export lookups and initialized shared root values
• bypass general lifecycle state transitions for non-shared parts with no imports or OnImportsSatisfied callbacks
• avoid LINQ and empty-array allocations while resolving constructor arguments through the existing reflection lifecycle
• add BenchmarkDotNet coverage for shared, non-shared, constructor, property, complex, and ImportMany activation
• document the investigation, JIT tradeoffs, compiled activation experiment, and staged hybrid-plan ideas in PERFORMANCE.md

This branch does not add expression-compiled constructors, setters, or activation plans. That experiment is preserved separately on perf/compiled-activation-plans and has no pull request.

Performance

Matched three-run medians from the IocPerformance scenarios. In the 1-thread variant, one thread performs 500,000 iterations. In the 2-thread variant, two threads share the same export provider and each performs 250,000 iterations concurrently. Each iteration resolves three exports, so both variants perform 1.5 million total resolutions. Values are total wall-clock time; the 2-thread variant measures concurrent throughput and contention rather than individual activation latency. main was measured at 26831df5; the PR column contains only the reduced, non-expression-compilation changes. The MEF2 control measurements differed by no more than 2.5% between runs.

The largest gains come from cached shared retrieval and the narrow lifecycle bypass for simple non-shared parts. Imported graphs retain 1.4-3.0x improvements from cached root lookup and lower constructor-argument overhead. Expression-compiled constructors, setters, and graph activation are not included in these PR results.

Validation

• Microsoft.VisualStudio.Composition.Tests: 2,064 net8.0 and 2,107 net472 tests passed
• benchmark project builds without warnings for net10.0 and net472
• all six activation benchmarks execute under both target frameworks
• no new expression compilation remains in the pull request diff

Validation pipeline

vs-mef validation build 15110883