refactor(config): generate scalar validation boundaries

libs/kaptan-config/src/lib.rs

@@ -22,4 +22,4 @@ pub use field::{
 pub use kaptan_config_derive::ConfigValidate;
 pub use lifecycle::{BeginError, CommitError, ConfigManager, ConfigState, SetError, StartupError};
 pub use storage::{Generation, ProfileStore};
-pub use validation::ValidateFields;
+pub use validation::{ConfigAggregate, ConfigSection, ValidateFields};

libs/kaptan-config/src/validation.rs

@@ -1,6 +1,6 @@
-//! Local scalar validation implemented by the configuration derive macro.
+//! Local scalar configuration sections implemented by the derive macro.
 
-use crate::ConfigViolation;
+use crate::{ConfigValue, ConfigViolation, FieldId, FieldWriteError};
 
 /// Validates local scalar invariants of one record.
 ///
@@ -11,3 +11,53 @@ pub trait ValidateFields {
     /// Rejects the first violated scalar or declared post-validation rule.
     fn validate_fields(&self) -> Result<(), ConfigViolation>;
 }
+
+/// A complete scalar configuration section generated from `#[config(...)]`
+/// field attributes.
+///
+/// Its metadata, default construction, reads, local writes, and scalar
+/// validation are all produced from the same field declarations. A section
+/// setter never commits a locally invalid value. Cross-section and capability
+/// invariants deliberately remain the aggregate candidate's responsibility.
+pub trait ConfigSection: ValidateFields {
+    /// Number of scalar descriptors generated for this section.
+    const FIELD_COUNT: usize;
+
+    /// Builds the section from derive-generated defaults after validating it.
+    fn default_section() -> Result<Self, ConfigViolation>
+    where
+        Self: Sized;
+
+    /// Returns one scalar by stable field identity.
+    fn field_value(&self, id: FieldId) -> Option<ConfigValue>;
+
+    /// Applies one scalar only when this section owns the ID and its local
+    /// generated validation accepts the new value.
+    fn set_field(&mut self, id: FieldId, value: ConfigValue) -> Result<(), FieldWriteError>;
+
+    /// Validates only generated scalar invariants. This intentionally omits
+    /// a derive `post` hook so staged multi-field edits can remain temporarily
+    /// cross-field-invalid until aggregate validation/commit.
+    fn validate_local_fields(&self) -> Result<(), ConfigViolation>;
+}
+
+/// An ordered aggregate of generated configuration sections.
+///
+/// The derive owns routing/default construction so adding a field to an
+/// existing section cannot silently bypass local validation. The aggregate
+/// owner still defines capability checks and immutable active construction.
+pub trait ConfigAggregate: ValidateFields {
+    /// Builds and fully validates all nested sections from their derived
+    /// defaults.
+    fn default_aggregate() -> Result<Self, ConfigViolation>
+    where
+        Self: Sized;
+
+    /// Returns one nested scalar by stable field identity.
+    fn field_value(&self, id: FieldId) -> Option<ConfigValue>;
+
+    /// Routes one scalar write to its owning generated section. The setter
+    /// applies local validation only, so a transaction may temporarily hold a
+    /// cross-field-invalid multi-edit candidate.
+    fn set_field(&mut self, id: FieldId, value: ConfigValue) -> Result<(), FieldWriteError>;
+}