perf(output): order jsonDiagnostic/jsonRelatedLocation pointer fields before scalars

internal/output.jsonDiagnostic interleaved Line/Column/
SourceStartLine/Deprecated (scalars) among its string/slice/pointer
fields; jsonRelatedLocation interleaved Line/Column before Message.
Same defect and same fix as internal/lint.Diagnostic and
pkg/mdsmith.Diagnostic: Go's GC ptrdata for a struct spans from
offset 0 through the last pointer-containing field, so a scalar
declared before that point sits inside the scanned span for nothing.
One of these is built per diagnostic on every `--format json` run
(CI pipelines, `mdsmith check --format json`).

Per docs/development/high-performance-go.md "Struct layout": order
fields large-to-small, group pointer fields first and scalars last.

Adds structlayout_test.go with the project's existing
structlayout.AssertPointerFieldsFirst pattern. Reordering the Go
struct fields changes encoding/json's emitted key order (Go encodes
struct fields in declaration order), so TestJSONFormatter_ExactOutput
is updated to match; JSON key order carries no semantic meaning and
no reference doc commits to a specific order, and the CLI's own e2e
JSON tests parse into a map rather than asserting exact text.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SNiF3CVZG2NCizbsTgur4V

Author: claude

internal/output/json.go

@@ -10,36 +10,45 @@ import (
 // JSONFormatter outputs diagnostics as a JSON array.
 type JSONFormatter struct{}
 
+// Fields are ordered pointer-containing (string/slice/pointer) first,
+// then scalar (int/bool) last, matching internal/lint.Diagnostic's
+// layout. Go's GC computes a struct's ptrdata as the offset through
+// the last pointer-containing field; one of these is built per
+// diagnostic on every `--format json` run. See
+// docs/development/high-performance-go.md "Struct layout".
 type jsonDiagnostic struct {
-	File            string           `json:"file"`
-	Line            int              `json:"line"`
-	Column          int              `json:"column"`
-	Rule            string           `json:"rule"`
-	Name            string           `json:"name"`
-	Severity        string           `json:"severity"`
-	Message         string           `json:"message"`
-	SourceLines     []string         `json:"source_lines,omitempty"`
-	SourceStartLine int              `json:"source_start_line,omitempty"`
-	Explanation     *jsonExplanation `json:"explanation,omitempty"`
-	// Deprecated and ReplacedBy mirror lint.Diagnostic's plan-136
-	// fields so CI scripts can route a deprecation warning without
-	// scanning the message body. Both are omitempty so non-
-	// deprecation diagnostics stay unchanged on the wire.
-	Deprecated bool   `json:"deprecated,omitempty"`
+	File        string           `json:"file"`
+	Rule        string           `json:"rule"`
+	Name        string           `json:"name"`
+	Severity    string           `json:"severity"`
+	Message     string           `json:"message"`
+	SourceLines []string         `json:"source_lines,omitempty"`
+	Explanation *jsonExplanation `json:"explanation,omitempty"`
+	// ReplacedBy mirrors lint.Diagnostic's plan-136 field so CI
+	// scripts can route a deprecation warning without scanning the
+	// message body. omitempty so non-deprecation diagnostics stay
+	// unchanged on the wire.
 	ReplacedBy string `json:"replaced_by,omitempty"`
 	// RelatedLocations mirrors lint.Diagnostic's plan-230 field so CI
 	// scripts can read the schema-constraint location without parsing
 	// the message. omitempty so diagnostics that carry none stay
 	// unchanged on the wire. The rule-doc URL is not emitted here — it
 	// is derivable from the `rule` field and is an editor (LSP) concern.
 	RelatedLocations []jsonRelatedLocation `json:"related_locations,omitempty"`
+
+	Line            int  `json:"line"`
+	Column          int  `json:"column"`
+	SourceStartLine int  `json:"source_start_line,omitempty"`
+	Deprecated      bool `json:"deprecated,omitempty"`
 }
 
+// File and Message (pointer-containing) precede Line and Column
+// (scalar) for the same GC-ptrdata reason as jsonDiagnostic above.
 type jsonRelatedLocation struct {
 	File    string `json:"file,omitempty"`
+	Message string `json:"message"`
 	Line    int    `json:"line,omitempty"`
 	Column  int    `json:"column,omitempty"`
-	Message string `json:"message"`
 }
 
 type jsonExplanation struct {

internal/output/json_test.go

@@ -170,12 +170,12 @@ func TestJSONFormatter_ExactOutput(t *testing.T) {
 	expected := `[
   {
     "file": "README.md",
-    "line": 10,
-    "column": 5,
     "rule": "MDS001",
     "name": "line-length",
     "severity": "error",
-    "message": "line too long (120 \u003e 80)"
+    "message": "line too long (120 \u003e 80)",
+    "line": 10,
+    "column": 5
   }
 ]
 `

internal/output/structlayout_test.go

@@ -0,0 +1,16 @@
+package output
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/jeduden/mdsmith/internal/structlayout"
+)
+
+func TestJSONDiagnostic_PointerFieldsPrecedeScalars(t *testing.T) {
+	structlayout.AssertPointerFieldsFirst(t, reflect.TypeOf(jsonDiagnostic{}))
+}
+
+func TestJSONRelatedLocation_PointerFieldsPrecedeScalars(t *testing.T) {
+	structlayout.AssertPointerFieldsFirst(t, reflect.TypeOf(jsonRelatedLocation{}))
+}