Summary

I added per-operation EventHook values so framework consumers can collect progress and diagnostic messages without writing them to stderr.

• Preserve existing initializer signatures and default stderr behavior.
• Propagate hooks through module builds, deferred documentation, file reads, and Clang header processing.
• Invoke file-read callbacks outside cache locks, including callbacks that read, replace, or invalidate cached contents.
• Document synchronous callback behavior and the boundaries: raw subprocess output and process-wide SourceKit restoration notifications remain unchanged.

Addresses #22.

Validation

• swift test --filter EventHookTests: 19 tests passed.
• swift build -c release: passed.
• SwiftLint 0.58.2 lint --strict: zero violations across 79 files.
• swift test: 95 tests, six failures. An untouched checkout of the same upstream commit has 76 tests and the same six failures on this host (Xcode 27.0 / Swift 6.4): ModuleTests.testCommandantDocs, ModuleTests.testCommandantDocsSPM, ModuleTests.testSpmDefaultModule, SourceKitTests.testIndex, SourceKitTests.testSwiftDeclarationAttributeKind, and SwiftDocsTests.testBicycle. These involve the legacy Xcode sample, SwiftPM build-manifest compatibility, and compiler-dependent fixtures.
• xcodebuild -scheme SourceKitten-Package -parallel-testing-enabled NO test -destination platform=macOS: built and began running tests, but stalled for over five minutes in the existing ClangTranslationUnitTests.testBasicObjectiveCDocs; I stopped the run. It is not a completed validation result.
• Linux and Bazel validation were not run locally.