docs: add schema-evolution.md — TypeBox Value.Diff/Patch/Cast for schema evolution

docs/architecture/forward-look.md

@@ -244,6 +244,13 @@ design in [metagraph-module.md](./metagraph-module.md):
    output is the same shape that a ujsx HostConfig would produce, but storage
    doesn't need ujsx to create it. The alignment is structural, not dependent.
 
+5. **Schemas-as-JSON enables `Value.Diff`/`Value.Patch`/`Value.Cast`** —
+   because TypeBox Modules serialize to JSON Schema, the TypeBox value system
+   can operate on schemas themselves (diff to detect changes, patch to update
+   stored schemas, cast to migrate data). This is not possible if schemas are
+   opaque builder objects or Drizzle column definitions. See
+   [schema-evolution.md](./schema-evolution.md).
+
 ## References
 
 - ujsx pointer system: `/workspace/@alkdev/ujsx/src/core/pointer.ts`
@@ -253,4 +260,5 @@ design in [metagraph-module.md](./metagraph-module.md):
 - dbtype module: `/workspace/@alkdev/dbtype/docs/architecture/module.md`
 - JPATH Module (JSONPath as TypeBox Module): `/workspace/research/typebox_research/ujsx/jpath.gen.ts`
 - jsonpathly source: `/workspace/jsonpathly/`
-- Module evolution spec: [metagraph-module.md](./metagraph-module.md)
+- Module evolution spec: [metagraph-module.md](./metagraph-module.md)
+- Schema evolution spec: [schema-evolution.md](./schema-evolution.md)