feat(macros): enhance Integrable derive with validation and versioning improvements

server/crates/arbiter-crypto/src/integrity.rs

@@ -1,6 +1,49 @@
 use crate::hashing::Hashable;
 
+/// Marks a struct as a participant in the database integrity system.
+///
+/// Implementors are protected by an HMAC-SHA256 MAC stored in the
+/// `integrity_envelope` table. The MAC is computed over:
+///
+/// ```text
+/// HMAC-SHA256(key, len(KIND) || KIND || len(entity_id) || entity_id || VERSION || SHA256(Hashable))
+/// ```
+///
+/// Both `KIND` and `VERSION` act as domain separators — they prevent a valid
+/// MAC for one entity type or schema version from being accepted for another.
+///
+/// # Deriving
+///
+/// Use `#[derive(Integrable)]` with the `#[integrable(kind = "...")]` attribute.
+/// `VERSION` is computed automatically as an FNV-1a hash of the struct's field
+/// names and types, so it changes whenever the schema changes without any manual
+/// bookkeeping.
+///
+/// ```rust,ignore
+/// #[derive(Hashable, Integrable)]
+/// #[integrable(kind = "operator_credentials")]
+/// pub struct OperatorCredentials {
+///     pub pubkey: PublicKey,
+/// }
+/// ```
+///
+/// # Upgrading schema
+///
+/// When fields are added, removed, or reordered, `VERSION` changes automatically.
+/// Existing MAC records in the database will return [`PayloadVersionMismatch`] on
+/// verification — this is the signal to re-sign all rows for this `KIND` as part
+/// of a migration.
+///
+/// [`PayloadVersionMismatch`]: crate::integrity::Integrable
 pub trait Integrable: Hashable {
+    /// Stable name of this entity type as stored in `integrity_envelope.entity_kind`.
+    ///
+    /// Must be a valid schema name: starts with a letter, contains only `[a-zA-Z0-9_]`,
+    /// and must be globally unique across all `Integrable` types in the system.
     const KIND: &'static str;
-    const VERSION: i32 = 1;
+
+    /// FNV-1a hash of the struct's field names and types at the time the derive
+    /// macro ran. Changes automatically when the schema changes, invalidating
+    /// existing MACs and signalling that a migration is required.
+    const VERSION: i32;
 }