docs: document XGo classfile var fields

AGENTS.md

@@ -107,6 +107,69 @@ func main() {
 
 3. **Error Handling**: If a file doesn't match any registered classfile pattern (wrong suffix), `BuildFile` returns "undefined" errors for DSL functions
 
+### Classfile `var` Fields
+
+In an XGo classfile, the first top-level `var (...)` declaration is the class
+fields declaration. These names compile into fields on the generated struct,
+not package-level variables. If the fields have initializers, XGo generates an
+`XGo_Init()` method and calls it before the classfile body runs.
+
+Source file `hello_llar.gox`:
+```gox
+var (
+    seen map[string]bool = {}
+)
+
+func mark(key string) {
+    seen[key] = true
+}
+
+id "DaveGamble/cJSON"
+fromVer "v1.0.0"
+
+onRequire (proj, deps) => {
+    mark "zlib"
+}
+```
+
+Generated Go code:
+```go
+package main
+
+import "github.com/goplus/llar/formula"
+
+type hello struct {
+    formula.ModuleF
+    seen map[string]bool
+}
+
+func (this *hello) mark(key string) {
+    this.seen[key] = true
+}
+
+func (this *hello) MainEntry() {
+    this.XGo_Init()
+    this.Id("DaveGamble/cJSON")
+    this.FromVer("v1.0.0")
+    this.OnRequire(func(proj *formula.Project, deps *formula.ModuleDeps) {
+        this.mark("zlib")
+    })
+}
+
+func (this *hello) Main() {
+    formula.Gopt_ModuleF_Main(this)
+}
+
+func (this *hello) XGo_Init() *hello {
+    this.seen = map[string]bool{}
+    return this
+}
+
+func main() {
+    new(hello).Main()
+}
+```
+
 ## Build & Test
 
 ```bash