Replaced all errors.New() with core.NewError() and errors.Is() with
core.Is() across error_test.go, error_example_test.go, utils_test.go.
The "errors" stdlib import is now zero across all test files.
Examples teach agents core.NewError() and core.Is() — not errors.New().
Dogfooding: Core's own tests use Core's own primitives.
Co-Authored-By: Virgil <virgil@lethean.io>
33 new examples across 8 dedicated files. Removed phantom CleanPath
(in RFC spec but never implemented — spec drift caught by examples).
545 tests total, 84.8% coverage. Every major primitive has compilable
examples that serve as test, documentation seed, and godoc content.
Co-Authored-By: Virgil <virgil@lethean.io>
