Add bounded diagnostic prefix encoding

docs/roadmap/implementation.md

@@ -492,6 +492,11 @@ invalid-byte runs, empty/small budgets, and truncation boundaries. Wiring into
 the public repair workflow follows in Stage 19; no interim alternative formatter
 is exposed or new contract created for a helper's exact allocation count.
 
+Completion: bounded UTF-8 normalization and JSON-prefix encoding retain only
+the useful candidate prefix and report complete versus truncated results.
+Focused encoding and existing formatter tests pass on Go 1.27.3; formatter
+wiring remains in the next unit.
+
 ## Stage 19: Construct Repair Feedback Without Whole-Input Serialization
 
 **Catalog:** 17, completion. **Files:** `formatRepairDiagnostics` and repair tests.

internal/usecase/repair_prefix.go

@@ -0,0 +1,48 @@
+package usecase
+
+import (
+	"encoding/json"
+	"strings"
+	"unicode/utf8"
+)
+
+// boundedDiagnosticJSON returns a JSON string prefix and whether it represents
+// the complete normalized input. It retains at most the useful prefix.
+func boundedDiagnosticJSON(value string, maxBytes int) ([]byte, bool) {
+	if maxBytes < 2 {
+		return nil, false
+	}
+	limit := maxBytes - 2
+	var prefix strings.Builder
+	used := 0
+	more := false
+	for i := 0; i < len(value); {
+		r, size := utf8.DecodeRuneInString(value[i:])
+		piece := value[i : i+size]
+		if r == utf8.RuneError && size == 1 {
+			piece = "\uFFFD"
+			i++
+			for i < len(value) {
+				r, size = utf8.DecodeRuneInString(value[i:])
+				if r != utf8.RuneError || size != 1 {
+					break
+				}
+				i++
+			}
+		} else {
+			i += size
+		}
+		if used+len(piece) > limit {
+			more = true
+			break
+		}
+		prefix.WriteString(piece)
+		used += len(piece)
+	}
+	normalized := prefix.String()
+	encoded, _ := json.Marshal(normalized)
+	if len(encoded) > maxBytes {
+		return truncateDiagnosticJSONValue(normalized, maxBytes), false
+	}
+	return encoded, !more
+}

internal/usecase/repair_prefix_test.go

@@ -0,0 +1,33 @@
+package usecase
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+func TestBoundedDiagnosticJSON(t *testing.T) {
+	for _, input := range []string{"", "plain", "<&>\n\"", "雪☃", "a\xff\xfe\xffb", "\xff\xfe" + strings.Repeat("z", 100)} {
+		for budget := 0; budget <= 30; budget++ {
+			encoded, complete := boundedDiagnosticJSON(input, budget)
+			if budget < 2 {
+				if encoded != nil || complete {
+					t.Fatalf("budget %d: %q %v", budget, encoded, complete)
+				}
+				continue
+			}
+			if len(encoded) > budget || !utf8.Valid(encoded) {
+				t.Fatalf("budget %d: %q", budget, encoded)
+			}
+			var decoded string
+			if err := json.Unmarshal(encoded, &decoded); err != nil {
+				t.Fatal(err)
+			}
+			normalized := strings.ToValidUTF8(input, "\uFFFD")
+			if !strings.HasPrefix(normalized, decoded) || complete != (decoded == normalized) {
+				t.Fatalf("input %q budget %d: %q complete=%v normalized=%q", input, budget, decoded, complete, normalized)
+			}
+		}
+	}
+}