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_parse.go

@@ -0,0 +1,111 @@
+package infra
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// MigrationParse parses a migration filename and its SQL content into a Migration.
+// The filename must follow the pattern NNN_name.sql (e.g. 001_create_users.sql).
+// The content must contain a -- +up marker; -- +down is optional but recommended.
+// Returns error if the filename format is invalid or the -- +up block is missing.
+func MigrationParse(filename, content string) (Migration, error) {
+	// Strip path prefix if any
+	base := filename
+	if idx := strings.LastIndex(base, "/"); idx >= 0 {
+		base = base[idx+1:]
+	}
+	if idx := strings.LastIndex(base, "\\"); idx >= 0 {
+		base = base[idx+1:]
+	}
+
+	// Remove .sql extension
+	name := base
+	if strings.HasSuffix(strings.ToLower(name), ".sql") {
+		name = name[:len(name)-4]
+	}
+
+	// Split on first underscore to get version and descriptive name
+	idx := strings.Index(name, "_")
+	if idx < 0 {
+		return Migration{}, fmt.Errorf("migration_parse: filename %q must follow pattern NNN_name.sql", filename)
+	}
+
+	versionStr := name[:idx]
+	descriptiveName := name[idx+1:]
+
+	version, err := strconv.Atoi(versionStr)
+	if err != nil || version <= 0 {
+		return Migration{}, fmt.Errorf("migration_parse: filename %q version %q must be a positive integer", filename, versionStr)
+	}
+
+	if descriptiveName == "" {
+		return Migration{}, fmt.Errorf("migration_parse: filename %q must have a non-empty descriptive name after the version", filename)
+	}
+
+	// Parse up/down blocks
+	upSQL, downSQL, err := parseMigrationBlocks(content)
+	if err != nil {
+		return Migration{}, fmt.Errorf("migration_parse: %w", err)
+	}
+
+	return Migration{
+		Version: version,
+		Name:    descriptiveName,
+		UpSQL:   upSQL,
+		DownSQL: downSQL,
+	}, nil
+}
+
+// parseMigrationBlocks splits SQL content by -- +up and -- +down markers.
+// Returns (upSQL, downSQL, error). The -- +up block is required.
+func parseMigrationBlocks(content string) (string, string, error) {
+	const markerUp = "-- +up"
+	const markerDown = "-- +down"
+
+	// Normalize line endings
+	content = strings.ReplaceAll(content, "\r\n", "\n")
+
+	upIdx := indexMarker(content, markerUp)
+	if upIdx < 0 {
+		return "", "", fmt.Errorf("missing -- +up marker in content")
+	}
+
+	downIdx := indexMarker(content, markerDown)
+
+	var upSQL, downSQL string
+
+	if downIdx < 0 {
+		// No down block
+		upSQL = strings.TrimSpace(content[upIdx+len(markerUp):])
+	} else if downIdx > upIdx {
+		// Normal order: up first, then down
+		upSQL = strings.TrimSpace(content[upIdx+len(markerUp) : downIdx])
+		downSQL = strings.TrimSpace(content[downIdx+len(markerDown):])
+	} else {
+		// Down before up — still valid, just unusual
+		downSQL = strings.TrimSpace(content[downIdx+len(markerDown) : upIdx])
+		upSQL = strings.TrimSpace(content[upIdx+len(markerUp):])
+	}
+
+	if upSQL == "" {
+		return "", "", fmt.Errorf("-- +up block is empty")
+	}
+
+	return upSQL, downSQL, nil
+}
+
+// indexMarker finds the index of a marker at the start of any line in content.
+func indexMarker(content, marker string) int {
+	lines := strings.Split(content, "\n")
+	pos := 0
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		if trimmed == marker {
+			return pos
+		}
+		pos += len(line) + 1 // +1 for the newline
+	}
+	return -1
+}