Deprecate default serde & add spl-collections

.changeset/odd-beans-bake.md

@@ -0,0 +1,33 @@
+---
+'@codama/renderers-rust': major
+---
+
+Removed default `serde` support and replaced `kaigan` with `spl-collections`
+
+**BREAKING CHANGES:**
+
+- Generated variable-sized string/vector wrappers now come from `spl-collections` instead of `kaigan`. If you have handwritten code that references these generated wrapper types, update those imports and usages after regenerating your client:
+  - `kaigan::types::RemainderStr` -> `spl_collections::TrailingStr`
+  - `kaigan::types::RemainderVec<T>` -> `spl_collections::TrailingVec<T>`
+  - `kaigan::types::U8PrefixString`, `U16PrefixString`, `U64PrefixString` -> `spl_collections::U8PrefixedStr`, `U16PrefixedStr`, `U64PrefixedStr`
+  - `kaigan::types::U8PrefixVec<T>`, `U16PrefixVec<T>`, `U64PrefixVec<T>` -> `spl_collections::U8PrefixedVec<T>`, `U16PrefixedVec<T>`, `U64PrefixedVec<T>`
+- `serde` is no longer part of the default or recommended generated client surface. The previous default derives on wrapper types were misleading because their `serde` representation does not match the Borsh/Wincode wire format.
+- If you still want `serde` derives for a separate JSON representation, you can opt in explicitly via `traitOptions` as shown below. This does not make the generated types serde-compatible with their Borsh/Wincode wire format, but you can define a handwritten implementation with your own serde mapping as needed.
+
+```diff
+  traitOptions: {
+    baseDefaults: [
+      'borsh::BorshSerialize',
+      'borsh::BorshDeserialize',
++     'serde::Serialize',
++     'serde::Deserialize',
+      'Clone',
+      'Debug',
+      'Eq',
+      'PartialEq',
+    ],
++   featureFlags: {
++     serde: ['serde::Serialize', 'serde::Deserialize'],
++   },
+  }
+```