perf(db): keyset pagination for revision prefixes

Keyset по id/ord вместо OFFSET; anti-join для RevisionDiff.

Co-authored-by: Cursor <[email protected]>

internal/store/pagination.go

@@ -0,0 +1,30 @@
+package store
+
+import (
+	"strconv"
+	"strings"
+)
+
+// ParsePrefixPageCursor decodes opaque cursors for revision prefix pagination.
+func ParsePrefixPageCursor(cursor string) (afterID *int64, offset int, useOffset bool) {
+	cursor = strings.TrimSpace(cursor)
+	if cursor == "" {
+		return nil, 0, false
+	}
+	if strings.HasPrefix(cursor, "o:") {
+		n, err := strconv.Atoi(strings.TrimPrefix(cursor, "o:"))
+		if err != nil || n < 0 {
+			return nil, 0, false
+		}
+		return nil, n, true
+	}
+	if n, err := strconv.ParseInt(cursor, 10, 64); err == nil && n >= 0 {
+		return &n, 0, false
+	}
+	return nil, 0, false
+}
+
+// FormatPrefixPageCursor encodes the keyset cursor (last row id or slice index).
+func FormatPrefixPageCursor(lastID int64) string {
+	return strconv.FormatInt(lastID, 10)
+}