feat: restore xattrs in directory restores

[rmanibus]

Closes #139

Summary

• replay stored extended attributes during direct-to-filesystem restores for files and directories on Unix platforms
• treat unsupported or permission-denied xattr writes as best-effort while still surfacing unexpected xattr restore failures
• add unit and restore-flow coverage for xattr replay behavior

Testing

• go test ./internal/engine ./cmd/cloudstic
• golangci-lint run ./internal/engine/... ./cmd/cloudstic/...

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/engine/restore.go	0.00%	2 Missing and 2 partials ⚠️

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