Implement reactive attributes for minimal DOM updates

Features:
- Track attributes that depend on signals (ternary expressions)
- Store element references in globals for reactive attributes
- Re-evaluate and update only changed attributes when signals change
- Similar to reactive conditionals but for attributes

Performance:
- Minimal DOM updates - only changed attributes are updated
- No full re-renders, no virtual DOM diffing
- Direct attribute updates via js_set_attribute

Example:
  class_='tab active' if tab_index == 0 else 'tab'

When tab_index changes, only the class attribute updates.

Implementation:
- Collection phase: scan for attributes with signal dependencies
- Render phase: store element references to globals
- Signal update: re-evaluate expression and update attribute

All tests passing (55 passed, 2 skipped)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_reactive_attributes.py

@@ -0,0 +1,99 @@
+"""Tests for reactive attributes that update when signals change."""
+
+import pytest
+from ritz.parser import parse_component
+from ritz.llvm_codegen_signals import generate_llvm_ir
+
+
+def test_parse_reactive_attribute():
+    """Test parsing attribute with signal dependency."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Toggle():
+    active = signal(1)
+
+    with Button(class_='active' if active else 'inactive'):
+        yield "Click"
+"""
+    ir = parse_component(source)
+
+    # Should have ternary expression in class attribute
+    with_stmt = [s for s in ir["body"] if s["type"] == "with"][0]
+    class_prop = with_stmt["element"]["props"]["class_"]
+    assert class_prop["type"] == "ifexp"
+    assert class_prop["test"]["id"] == "active"
+
+
+def test_reactive_attribute_infrastructure():
+    """Test that reactive attribute tracking is generated."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Toggle():
+    active = signal(1)
+
+    def toggle():
+        active = 0 if active else 1
+
+    with Button(class_='btn-active' if active else 'btn-inactive', onclick=toggle):
+        yield "Toggle"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Should have storage for the button element
+    assert "@reactive_attr_" in llvm_ir or "@elem_" in llvm_ir
+
+    # Should update attribute when signal changes in toggle()
+    # The toggle function should re-evaluate the class expression and update
+    assert "js_set_attribute" in llvm_ir
+
+
+def test_multiple_reactive_attributes():
+    """Test multiple elements with reactive attributes."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Tabs():
+    tab = signal(0)
+
+    def set_tab(value):
+        tab = value
+
+    with Ul():
+        with Li(class_='active' if tab == 0 else ''):
+            yield "Tab 0"
+
+        with Li(class_='active' if tab == 1 else ''):
+            yield "Tab 1"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Both Li elements should have reactive attributes
+    # When tab signal changes, both should update
+    assert llvm_ir.count("js_set_attribute") >= 2
+
+
+def test_reactive_attribute_with_comparison():
+    """Test reactive attribute with comparison in ternary."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Counter():
+    count = signal(5)
+
+    with Div(class_='high' if count > 10 else 'low'):
+        yield "Value"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Should emit comparison when evaluating class
+    assert "icmp" in llvm_ir
+    assert "select" in llvm_ir