feat(core): add csv dep + import error variants

Adds csv = "1" to relicario-core; introduces
ImportCsvHeader and ImportCsvFormat. Foundation for the
import_lastpass module landing in Task 2.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/Cargo.toml

@@ -29,5 +29,6 @@ getrandom = "0.2"
 zstd = { version = "0.13", default-features = false }
 tar = { version = "0.4", default-features = false }
 base64 = "0.22"
+csv = "1"
 
 [dev-dependencies]