Catch undefined variables and functions before compilation

Rugo now reports undefined variable and function errors instantly
instead of waiting for the Go compiler to fail. Error messages
include the exact source file and line number.

Examples of errors now caught early:

  script.rugo:3: undefined variable 'xyz'
  script.rugo:5: undefined function helper.nonexistent

This also validates namespaced calls (ns.func()) against the
actual functions exported by required modules, stdlib modules,
and Go bridge packages.

Author: rubiojr

ast/cache.go

@@ -11,7 +11,7 @@ import (
 	"github.com/rubiojr/rugo/parser"
 )
 
-//go:embed nodes.go walker.go preprocess.go parse.go lower.go transform.go factory.go implicit_return.go
+//go:embed check.go nodes.go walker.go preprocess.go parse.go lower.go transform.go factory.go implicit_return.go
 var Sources embed.FS
 
 // goModTemplate is the go.mod for the cached AST module.

ast/check.go

@@ -0,0 +1,20 @@
+package ast
+
+// Check validates an AST without modifying it.
+type Check interface {
+	Name() string
+	Check(prog *Program) error
+}
+
+// CheckChain runs checks in order, stopping at the first error.
+type CheckChain []Check
+
+// Run executes each check in sequence. Returns nil if all pass.
+func (cc CheckChain) Run(prog *Program) error {
+	for _, c := range cc {
+		if err := c.Check(prog); err != nil {
+			return err
+		}
+	}
+	return nil
+}