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/bun/identity_column_test.go

@@ -0,0 +1,49 @@
+package bun
+
+import (
+	"strings"
+	"testing"
+
+	"git.warky.dev/wdevs/relspecgo/pkg/models"
+)
+
+func TestBuildBunTag_IdentityColumn(t *testing.T) {
+	mapper := NewTypeMapper("", "")
+
+	build := func(pk bool, identity bool, generation string) *models.Column {
+		col := models.InitColumn("id", "users", "public")
+		col.Type = "bigint"
+		col.IsPrimaryKey = pk
+		col.Identity = identity
+		col.IdentityGeneration = generation
+		return col
+	}
+
+	tests := []struct {
+		name       string
+		column     *models.Column
+		wantFlag   bool
+		wantMarker bool
+	}{
+		{"always identity non-pk is write-blocked", build(false, true, "ALWAYS"), true, true},
+		{"always identity pk is only marked", build(true, true, "ALWAYS"), false, true},
+		{"by default identity is writable", build(false, true, "BY DEFAULT"), false, false},
+		{"non-identity is untouched", build(false, false, ""), false, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tag := mapper.BuildBunTag(tt.column, nil)
+			has := func(s string) bool {
+				for _, p := range strings.Split(tag, ",") {
+					if p == s {
+						return true
+					}
+				}
+				return false
+			}
+			if has("scanonly") != tt.wantFlag || has("identity") != tt.wantMarker {
+				t.Errorf("tag %q: scanonly/identity = %v/%v, want %v/%v", tag, has("scanonly"), has("identity"), tt.wantFlag, tt.wantMarker)
+			}
+		})
+	}
+}