Implement RFC 0015 filemeta path normalization

[rmanibus]

Summary

• stop persisting FileMeta.Paths in newly written filemeta objects and omit empty paths from JSON
• add shared derived-path helpers so restore, diff, and incremental backup path reconstruction do not depend on stored paths
• add mixed old/new snapshot compatibility coverage and mark RFC 0015 as implemented

Closes #131.

Testing

• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./internal/engine ./internal/core
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./cmd/cloudstic
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./... (fails in sandbox at internal/retry.TestClassifyHTTPResponse_Integration because httptest cannot bind a local port: listen tcp6 [::1]:0: bind: operation not permitted)

[codecov]

Codecov Report

❌ Patch coverage is 86.41975% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/engine/diff.go	80.00%	3 Missing and 3 partials ⚠️
internal/engine/filemeta_paths.go	89.18%	2 Missing and 2 partials ⚠️
internal/engine/backup_scan.go	87.50%	1 Missing ⚠️

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