docs(core): add "type" tag-collision invariant to ItemCore

Reviewer note: flatten semantics of serde tag = "type" means no *Core
struct may ever use "type" as a field name.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/item_types/mod.rs

@@ -35,6 +35,9 @@ pub enum ItemType {
     Totp,
 }
 
+// INVARIANT: no *Core struct may have a field serialized as "type" —
+// that key is reserved for serde's internal tag. Use "kind" for
+// type-discriminant fields within core structs (CardKind, TotpKind).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ItemCore {