Replace Cache with Event object in callbacks (#45)

* Replace Cache with Event object in callbacks

Introduces FixtureKit::Event, which wraps the fixture and exposes
identifier (cache key) and path (source location of the definition
block). This gives callback subscribers the definition file path,
which was the original motivation for the change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Rename Definition#source_location to Definition#path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove unnecessary safe navigation in Definition#path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>