feat: mark generated and identity columns across writers and DBML

- bun: scanonly+generated for GENERATED columns; scanonly+identity for ALWAYS identity (PK gets identity only)
- gorm: <-:false+generated / identity, same primary key rule
- dbml: write GENERATED/IDENTITY column notes and read them back into the model
- pgsql: emit generation/identity clauses in migration create-table and add-column, no DEFAULT
- readmes updated

pkg/writers/dbml/README.md

@@ -111,6 +111,8 @@ Table table_name {
 - `unique` - Unique constraint
 - `default: value` - Default value
 - `note: 'text'` - Column note
+- `note: 'GENERATED ALWAYS AS (expr) STORED'` - Generated column (`Generated`, `GenerationExpression`)
+- `note: 'GENERATED ALWAYS|BY DEFAULT AS IDENTITY'` - Identity column (`Identity`, `IdentityGeneration`)
 
 ### Relationships
 ```dbml