Fix reactive conditional rendering with proper branch swapping

Implements efficient reactive conditionals that pre-render branches once
and swap DOM nodes without recreation, enabling fast switching with
thousands of elements.

Key Fixes:
- Runtime now calls reevaluate_conditional_*() on signal changes
- Match/case swap logic uses phi nodes to load correct old case
- Previously hardcoded case_0 as old value, causing accumulation
- Branch render functions now wrap all content in container div

Implementation:
- Pre-render both if/else branches at init
- Store roots in globals (@cond_N_then_root, @cond_N_else_root)
- On signal change, swap DOM nodes via js_replace_at_marker()
- Match statements use switch/phi pattern to select old case

Tests:
- Added test_reactive_conditionals.py (5 new tests)
- Updated test_llvm_codegen_conditional.py for reactive infrastructure
- All 35 tests passing

Performance: No re-rendering on switches, perfect for dynamic content.

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

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

tests/test_reactive_conditionals.py

@@ -0,0 +1,169 @@
+"""Tests for reactive conditional rendering with signal updates."""
+
+import pytest
+from ritz.parser import parse_component
+from ritz.llvm_codegen_signals import generate_llvm_ir
+
+
+def test_reactive_conditional_infrastructure():
+    """Test that reactive conditional infrastructure is generated."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Toggle():
+    visible = signal(1)
+
+    if visible:
+        with P():
+            yield "Visible"
+    else:
+        with P():
+            yield "Hidden"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Should have marker global
+    assert "@cond_0_marker = global i32 0" in llvm_ir
+
+    # Should have current state global
+    assert "@cond_0_current = global i32 0" in llvm_ir
+
+    # Should have root globals for both branches
+    assert "@cond_0_then_root = global i32 0" in llvm_ir
+    assert "@cond_0_else_root = global i32 0" in llvm_ir
+
+    # Should have init function
+    assert "define void @_init_conditional_0()" in llvm_ir
+
+    # Should have reevaluate function (for signal updates)
+    assert "define void @reevaluate_conditional_0()" in llvm_ir
+
+    # Should have branch render functions
+    assert "define i32 @_render_cond_0_then()" in llvm_ir
+    assert "define i32 @_render_cond_0_else()" in llvm_ir
+
+
+def test_reactive_conditional_init_logic():
+    """Test that init function pre-renders both branches."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Toggle():
+    visible = signal(1)
+
+    if visible:
+        with P():
+            yield "Shown"
+    else:
+        with P():
+            yield "Hidden"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Init should call both render functions
+    assert "call i32 @_render_cond_0_then()" in llvm_ir
+    assert "call i32 @_render_cond_0_else()" in llvm_ir
+
+    # Init should store both roots
+    assert "store i32" in llvm_ir and "@cond_0_then_root" in llvm_ir
+    assert "store i32" in llvm_ir and "@cond_0_else_root" in llvm_ir
+
+    # Init should insert active branch at marker
+    assert "call void @js_replace_at_marker" in llvm_ir
+
+
+def test_reactive_conditional_swap_logic():
+    """Test that reevaluate function swaps branches when condition changes."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Toggle():
+    visible = signal(0)
+
+    if visible:
+        with P():
+            yield "On"
+    else:
+        with P():
+            yield "Off"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Reevaluate should re-evaluate the condition
+    assert "reevaluate_conditional_0" in llvm_ir
+
+    # Should check if condition changed
+    assert "icmp ne" in llvm_ir
+
+    # Should have swap logic
+    assert "swap:" in llvm_ir or "swap_to_then:" in llvm_ir
+    assert "swap_to_then:" in llvm_ir
+    assert "swap_to_else:" in llvm_ir
+
+
+def test_multiple_reactive_conditionals():
+    """Test multiple if statements in same component get unique IDs."""
+    source = """
+from ritz.syntax import *
+
+@component
+def MultiToggle():
+    flag1 = signal(1)
+    flag2 = signal(0)
+
+    if flag1:
+        with P():
+            yield "First on"
+
+    if flag2:
+        with P():
+            yield "Second on"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Should have two separate conditionals
+    assert "@cond_0_marker" in llvm_ir
+    assert "@cond_1_marker" in llvm_ir
+
+    # Should have init functions for both
+    assert "define void @_init_conditional_0()" in llvm_ir
+    assert "define void @_init_conditional_1()" in llvm_ir
+
+    # Should have reevaluate functions for both
+    assert "define void @reevaluate_conditional_0()" in llvm_ir
+    assert "define void @reevaluate_conditional_1()" in llvm_ir
+
+
+def test_reactive_conditional_with_comparison():
+    """Test reactive conditional with comparison operator."""
+    source = """
+from ritz.syntax import *
+
+@component
+def Counter():
+    count = signal(0)
+
+    if count > 5:
+        with P():
+            yield "High"
+    else:
+        with P():
+            yield "Low"
+"""
+    ir = parse_component(source)
+    llvm_ir = generate_llvm_ir(ir)
+
+    # Should generate icmp for comparison
+    assert "icmp" in llvm_ir
+    assert "sgt" in llvm_ir  # signed greater than
+
+    # Should still have reactive infrastructure
+    assert "@_init_conditional_0()" in llvm_ir
+    assert "@reevaluate_conditional_0()" in llvm_ir