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/src/error.rs

@@ -51,6 +51,17 @@ pub enum RelicarioError {
     #[error("backup envelope schema v{found}; this relicario reads v{expected}")]
     BackupSchemaMismatch { found: u32, expected: u32 },
 
+    /// CSV header doesn't match the LastPass column layout.
+    #[error("unrecognized CSV header — expected LastPass export format ({0})")]
+    ImportCsvHeader(String),
+
+    /// CSV body could not be parsed (mismatched quoting, encoding, etc.).
+    /// Per-row record errors that the importer recovers from become
+    /// `ImportWarning` entries — this variant is reserved for failures
+    /// that abort the whole import.
+    #[error("CSV parse failed: {0}")]
+    ImportCsvFormat(String),
+
     /// An item was looked up by ID but does not exist in the manifest.
     #[error("item not found: {0}")]
     ItemNotFound(String),
@@ -156,4 +167,15 @@ mod tests {
         let s = format!("{}", schema);
         assert!(s.contains("v2") && s.contains("v1"));
     }
+
+    #[test]
+    fn import_errors_carry_useful_messages() {
+        let h = RelicarioError::ImportCsvHeader("missing 'name' column".into());
+        assert!(format!("{}", h).contains("LastPass"));
+        assert!(format!("{}", h).contains("missing 'name'"));
+
+        let f = RelicarioError::ImportCsvFormat("unterminated quote at line 12".into());
+        assert!(format!("{}", f).contains("CSV parse failed"));
+        assert!(format!("{}", f).contains("unterminated quote"));
+    }
 }