fix: avoid variable shadowing in nested mixin calls with same parameter name

When a mixin calls another mixin passing a variable with the same name
as the parameter (e.g., +alert(message) where alert has param message),
skip generating redundant const declaration since the variable is already
in scope.

Also adds missing alert.pug mixin for demo project.

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .pugz,
-    .version = "0.1.5",
+    .version = "0.1.6",
     .fingerprint = 0x822db0790e17621d, // Changing this has security and trust implications.
     .minimum_zig_version = "0.15.2",
     .dependencies = .{},

examples/demo/views/mixins/alert.pug

@@ -0,0 +1,5 @@
+mixin alert(message)
+  div.alert(role="alert" class!=attributes.class)
+    svg(xmlns="http://www.w3.org/2000/svg" class="h-6 w-6 shrink-0 stroke-current" fill="none" viewBox="0 0 24 24")
+      path(stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 14l2-2m0 0l2-2m-2 2l-2-2m2 2l2 2m7-2a9 9 0 11-18 0 9 9 0 0118 0z")
+    span= message

examples/demo/views/mixins/alert_error.pug

@@ -0,0 +1,2 @@
+mixin alert_error(message)
+  +alert(message)(class="alert-error")

examples/demo/views/page-a.pug

@@ -24,3 +24,5 @@ block content
     br
     +input_text("lastName", "Last Name", "last name")
     submit sumit
+    if error
+      +alert_error(error)

src/build_templates.zig

@@ -1336,21 +1336,26 @@ const Compiler = struct {
             var ident_buf: [64]u8 = undefined;
             const safe_param = escapeIdent(param, &ident_buf);
 
-            try self.writeIndent();
             if (i < call.args.len) {
                 // Argument provided
                 const arg = call.args[i];
                 // Check if it's a string literal
                 if (arg.len >= 2 and (arg[0] == '"' or arg[0] == '\'')) {
+                    try self.writeIndent();
                     try self.writer.print("const {s} = {s};\n", .{ safe_param, arg });
                 } else {
                     // It's a variable reference
                     var accessor_buf: [512]u8 = undefined;
                     const accessor = self.buildAccessor(arg, &accessor_buf);
-                    try self.writer.print("const {s} = {s};\n", .{ safe_param, accessor });
+                    // Skip declaration if accessor equals param name (already in scope)
+                    if (!std.mem.eql(u8, accessor, safe_param)) {
+                        try self.writeIndent();
+                        try self.writer.print("const {s} = {s};\n", .{ safe_param, accessor });
+                    }
                 }
             } else if (i < mixin_def.defaults.len) {
                 // Use default value
+                try self.writeIndent();
                 if (mixin_def.defaults[i]) |default| {
                     try self.writer.print("const {s} = {s};\n", .{ safe_param, default });
                 } else {
@@ -1358,6 +1363,7 @@ const Compiler = struct {
                 }
             } else {
                 // No value - use empty string
+                try self.writeIndent();
                 try self.writer.print("const {s} = \"\";\n", .{safe_param});
             }
         }