feat: preserve metadata ordering in dir restores

[rmanibus]

Closes #104

Summary

• make direct-to-filesystem restores rerun-safe by skipping existing file collisions with warnings instead of truncating them
• defer directory metadata and flags until the end of the restore so child writes cannot clobber final timestamps or permissions
• add Unix metadata replay helpers for mode, ownership, timestamps, xattrs, and flags, plus warning reporting in restore summaries

Notes

• this builds on the xattr replay work already merged for RFC 0006
• birth time replay is still best-effort and remains unimplemented on macOS/Linux

Testing

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

[codecov]

Codecov Report

❌ Patch coverage is 61.71875% with 49 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/engine/restore.go	67.85%	10 Missing and 8 partials ⚠️
internal/engine/restore_metadata_unix.go	42.85%	8 Missing and 8 partials ⚠️
internal/engine/restore_metadata_linux.go	52.17%	7 Missing and 4 partials ⚠️
internal/engine/restore_xattrs_unix.go	81.81%	1 Missing and 1 partial ⚠️
pkg/source/local_source_xattr_unix.go	75.00%	1 Missing and 1 partial ⚠️

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