fix: merge multiple class attributes in parser (single fix location)

- parser.zig: collect and merge all class values (shorthand and parenthesized) into single attribute
- Filter out empty, null, undefined class values during parsing
- Reverted redundant merging logic from codegen.zig, template.zig, zig_codegen.zig
- Added documentation about shared AST consumers relationship

build.zig.zon

@@ -1,6 +1,6 @@
 .{
     .name = .pugz,
-    .version = "0.3.5",
+    .version = "0.3.6",
     .fingerprint = 0x822db0790e17621d, // Changing this has security and trust implications.
     .minimum_zig_version = "0.15.2",
     .dependencies = .{},