feat: funciones impuras migration_create, migration_up, migration_down, migration_status

Fase 2 del issue 0015. MigrationCreate (crea archivo .sql template con version
auto-calculada), MigrationUp (aplica migraciones pendientes en transacciones
individuales), MigrationDown (revierte ultimas N via down_sql de _migrations),
MigrationGetStatus (cruza disco con BD, detecta orphaned). Tests de integracion:
ciclo completo create->up->status->down->status. 26 tests, todos pasan.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/migration_create_test.go

@@ -0,0 +1,78 @@
+package infra
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestMigrationCreate(t *testing.T) {
+	t.Run("directorio vacio crea archivo con version 001", func(t *testing.T) {
+		dir := t.TempDir()
+		path, err := MigrationCreate(dir, "create_users")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		base := filepath.Base(path)
+		if base != "001_create_users.sql" {
+			t.Errorf("filename: got %q, want %q", base, "001_create_users.sql")
+		}
+
+		content, err := os.ReadFile(path)
+		if err != nil {
+			t.Fatalf("cannot read created file: %v", err)
+		}
+		if !strings.Contains(string(content), "-- +up") {
+			t.Errorf("file missing -- +up marker")
+		}
+		if !strings.Contains(string(content), "-- +down") {
+			t.Errorf("file missing -- +down marker")
+		}
+	})
+
+	t.Run("directorio con migraciones existentes calcula siguiente version", func(t *testing.T) {
+		dir := t.TempDir()
+
+		// Create existing files
+		os.WriteFile(filepath.Join(dir, "001_create_users.sql"), []byte("-- +up\nCREATE TABLE users (id TEXT);\n-- +down\nDROP TABLE users;\n"), 0o644)
+		os.WriteFile(filepath.Join(dir, "002_add_email.sql"), []byte("-- +up\nALTER TABLE users ADD COLUMN email TEXT;\n-- +down\n\n"), 0o644)
+
+		path, err := MigrationCreate(dir, "add_index")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		base := filepath.Base(path)
+		if base != "003_add_index.sql" {
+			t.Errorf("filename: got %q, want %q", base, "003_add_index.sql")
+		}
+	})
+
+	t.Run("nombre invalido retorna error", func(t *testing.T) {
+		dir := t.TempDir()
+		_, err := MigrationCreate(dir, "123invalid")
+		if err == nil {
+			t.Fatal("expected error for invalid name, got nil")
+		}
+	})
+
+	t.Run("directorio inexistente se crea automaticamente", func(t *testing.T) {
+		base := t.TempDir()
+		dir := filepath.Join(base, "nested", "migrations")
+
+		path, err := MigrationCreate(dir, "init")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if _, err := os.Stat(dir); os.IsNotExist(err) {
+			t.Errorf("directory was not created: %s", dir)
+		}
+
+		if filepath.Dir(path) != dir {
+			t.Errorf("file not in expected dir: %s", path)
+		}
+	})
+}