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_validate.go

@@ -0,0 +1,71 @@
+package infra
+
+import (
+	"fmt"
+	"sort"
+)
+
+// MigrationValidate checks a slice of migrations for consistency errors.
+// It verifies that:
+//   - Versions are sequential starting from 1 with no gaps (1, 2, 3...)
+//   - No duplicate versions exist
+//   - Each migration has non-empty UpSQL
+//   - Each migration has a non-empty Name
+//
+// Returns a slice of human-readable error strings. An empty slice means all
+// migrations are valid. The function does not mutate the input slice.
+func MigrationValidate(migrations []Migration) []string {
+	var errs []string
+
+	if len(migrations) == 0 {
+		return errs
+	}
+
+	// Work on a sorted copy to detect gaps and duplicates
+	sorted := make([]Migration, len(migrations))
+	copy(sorted, migrations)
+	sort.Slice(sorted, func(i, j int) bool {
+		return sorted[i].Version < sorted[j].Version
+	})
+
+	// Check individual fields and collect duplicates
+	seen := make(map[int]int) // version -> count
+	for _, m := range sorted {
+		seen[m.Version]++
+
+		if m.Name == "" {
+			errs = append(errs, fmt.Sprintf("version %d has empty name", m.Version))
+		}
+		if m.UpSQL == "" {
+			errs = append(errs, fmt.Sprintf("version %d (%s) has empty up_sql", m.Version, m.Name))
+		}
+	}
+
+	// Report duplicates
+	for v, count := range seen {
+		if count > 1 {
+			errs = append(errs, fmt.Sprintf("duplicate version %d appears %d times", v, count))
+		}
+	}
+
+	// Check sequential numbering starting from 1 (no gaps)
+	// Build unique sorted versions
+	versions := make([]int, 0, len(seen))
+	for v := range seen {
+		versions = append(versions, v)
+	}
+	sort.Ints(versions)
+
+	if len(versions) > 0 && versions[0] != 1 {
+		errs = append(errs, fmt.Sprintf("versions must start at 1, got %d", versions[0]))
+	}
+
+	for i := 1; i < len(versions); i++ {
+		expected := versions[i-1] + 1
+		if versions[i] != expected {
+			errs = append(errs, fmt.Sprintf("gap in versions: missing %d (have %d then %d)", expected, versions[i-1], versions[i]))
+		}
+	}
+
+	return errs
+}