feat: tipos Migration/MigrationStatus y funciones puras migration_parse + migration_validate

Fase 1 del issue 0015. Tipos Go en functions/infra/migration.go con metadata en
types/infra/. Funciones puras: MigrationParse (parsea filename NNN_name.sql +
bloques -- +up/-- +down) y MigrationValidate (verifica secuencia, huecos,
duplicados, bloques vacios). 16 tests, todos pasan.

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

functions/infra/migration.go

@@ -0,0 +1,20 @@
+package infra
+
+import "time"
+
+// Migration represents a migration parsed from a .sql file.
+type Migration struct {
+	Version   int       // sequential number (1, 2, 3...)
+	Name      string    // descriptive name (create_entities, add_status_column)
+	UpSQL     string    // SQL block to apply the migration
+	DownSQL   string    // SQL block to revert the migration
+	AppliedAt time.Time // zero value if not yet applied
+}
+
+// MigrationStatus represents the state of a migration relative to a database.
+type MigrationStatus struct {
+	Version   int       // sequential number
+	Name      string    // descriptive name
+	Applied   bool      // true if already applied in the database
+	AppliedAt time.Time // when it was applied (zero value if pending)
+}