fix: Phase 3 — #25,28,29,41 token/AST/parser cleanup

Files modified (4):
  compiler/token/token.go — #25: Replace hand-rolled itoa with strconv.Itoa
    Fixes math.MinInt overflow bug in original implementation
  compiler/ast/ast.go — #29: Fix VarDecl.End() returning last var position
    Was returning Pos() (useless span info)
  compiler/parser/stmtreg.go — #28: Eliminate all 7 token array mutations
    rewriteAsIdent() modifies p.current only, not the token array
    Prevents backtracking corruption and improves safety
  compiler/lexer/lexer.go — Already clean from Phase 2

Issues resolved: #25 (MEDIUM), #28 (MEDIUM), #29 (MEDIUM), #41 partial (LOW)
Total fixed: 29/53

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compiler/ast/ast.go

@@ -117,7 +117,12 @@ type VarDecl struct {
 }
 
 func (d *VarDecl) Pos() token.Position { return d.DeclPos }
-func (d *VarDecl) End() token.Position { return d.DeclPos }
+func (d *VarDecl) End() token.Position {
+	if len(d.Vars) > 0 {
+		return d.Vars[len(d.Vars)-1].NamePos
+	}
+	return d.DeclPos
+}
 func (d *VarDecl) declNode()           {}
 func (d *VarDecl) stmtNode()           {} // PRIVATE/PUBLIC can appear as statements