fix: canonicalize relative local source paths

[rmanibus]

Closes #145

Summary

• avoid deriving bogus volume-relative local source paths when the resolved source path is not actually under the detected mount root
• keep local:./ and similar relative roots persisted as canonical absolute paths when no safe relative path exists
• add regression coverage for the broken relative-root case and the root-containment helper

Testing

• go test ./pkg/source ./cmd/cloudstic

[codecov]

Codecov Report

❌ Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/source/local_source.go	58.82%	4 Missing and 3 partials ⚠️

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