internal: simplify structpath to eager initialize (#3594)

## Changes
Simplify structpath by removing lazy initialization.

## Why
Easier to extend this package.

## Tests
Existing tests.

Ran benchmarks on structdiff and structwalk, there are no noticeable
changes.

Author: denik

libs/structdiff/structpath/path.go

@@ -9,18 +9,17 @@ import (
 )
 
 const (
-	tagStruct           = -1
-	tagMapKey           = -2
-	tagUnresolvedStruct = -3
-	tagAnyKey           = -4
-	tagAnyIndex         = -5
+	tagStruct   = -1
+	tagMapKey   = -2
+	tagAnyKey   = -4
+	tagAnyIndex = -5
 )
 
 // PathNode represents a node in a path for struct diffing.
 // It can represent struct fields, map keys, or array/slice indices.
 type PathNode struct {
 	prev      *PathNode
-	jsonTag   structtag.JSONTag // For lazy JSON key resolution
+	jsonTag   structtag.JSONTag // For JSON key resolution
 	bundleTag structtag.BundleTag
 	key       string // Computed key (JSON key for structs, string key for maps, or Go field name for fallback)
 	// If index >= 0, the node specifies a slice/array index in index.
@@ -74,23 +73,10 @@ func (p *PathNode) AnyIndex() bool {
 	return p.index == tagAnyIndex
 }
 
-func (p *PathNode) resolveField() {
-	if p.index == tagUnresolvedStruct {
-		// Lazy resolve JSON key for struct fields
-		jsonName := p.jsonTag.Name()
-		if jsonName != "" {
-			p.key = jsonName
-		}
-		// If jsonName is empty, key already contains the Go field name as fallback
-		p.index = tagStruct
-	}
-}
-
 func (p *PathNode) Field() (string, bool) {
 	if p == nil {
 		return "", false
 	}
-	p.resolveField()
 	if p.index == tagStruct {
 		return p.key, true
 	}
@@ -125,17 +111,22 @@ func NewMapKey(prev *PathNode, key string) *PathNode {
 }
 
 // NewStructField creates a new PathNode for a struct field.
-// The jsonTag is used for lazy JSON key resolution, and fieldName is used as fallback.
+// The jsonTag is used for JSON key resolution, and fieldName is used as fallback.
 func NewStructField(prev *PathNode, tag reflect.StructTag, fieldName string) *PathNode {
 	jsonTag := structtag.JSONTag(tag.Get("json"))
 	bundleTag := structtag.BundleTag(tag.Get("bundle"))
 
+	key := fieldName
+	if name := jsonTag.Name(); name != "" {
+		key = name
+	}
+
 	return &PathNode{
 		prev:      prev,
 		jsonTag:   jsonTag,
 		bundleTag: bundleTag,
-		key:       fieldName,
-		index:     tagUnresolvedStruct,
+		key:       key,
+		index:     tagStruct,
 	}
 }
 
@@ -167,8 +158,6 @@ func (p *PathNode) String() string {
 		return p.prev.String() + "[*]"
 	}
 
-	p.resolveField()
-
 	if p.index == tagStruct {
 		return p.prev.String() + "." + p.key
 	}
@@ -199,8 +188,6 @@ func (p *PathNode) DynPath() string {
 		return p.prev.DynPath() + "[*]"
 	}
 
-	p.resolveField()
-
 	prev := p.prev.DynPath()
 	if prev == "" {
 		return p.key