Implement self-hosted debug console and @external decorator

This commit completes the self-hosting of Ritz's debug console and
introduces the @external decorator for elegant JavaScript bridge functions.

## Major Features

### 1. Self-Hosted Debug Console (DebugConsole.py)
- Entire debug console UI written in Ritz itself
- Signal-based state management for toggle functionality
- Displays runtime stats (memory, signals, DOM nodes, logs)
- Live reactive updates when signals change in main app
- Keyboard shortcut support (Ctrl+`)

### 2. @external Decorator for JavaScript Bridges
- Pythonic decorator syntax: @external or @bridge
- Parser automatically recognizes and tracks external functions
- Codegen emits proper LLVM declarations
- No more hacky string checks (startswith("js_"))
- Clean, declarative bridge function definitions

Example:
```python
@external
def js_toggle_debug_console():
    pass

def toggle():
    js_toggle_debug_console()  # Clean call to JS bridge
```

### 3. Enhanced RitzLoader Debug Mode
- `new RitzLoader({ debug: true })` auto-loads DebugConsole.wasm
- Keyboard shortcut registration (Ctrl+` toggle)
- Runtime stats injection (memory, signals, DOM nodes)
- Automatic status element hiding when debug enabled
- Debug console isolated in separate WASM runtime

## Bug Fixes

- Fixed reactive text node scoping (prevent global name collisions)
- Fixed CSS specificity for debug console status (use #ritz-debug)
- Fixed signal cross-contamination between runtimes (scope to rootElement)
- Fixed parser to handle expression statements in function bodies
- Fixed conditional branch rendering (append children to wrapper divs)

## Infrastructure Improvements

- Added H4 and Pre elements to syntax.py
- Parser handles @external decorator on top-level functions
- Codegen pre-collects external functions before emitting declarations
- Runtime scopes querySelector to prevent cross-runtime interference
- Debug stats update reactively on every signal change

## Files Added/Modified

New files:
- examples/runtime/DebugConsole.py - Self-hosted debug UI
- examples/runtime/DebugConsole.wasm - Compiled debug console (1.8KB)
- PHASE4_CLEANUP.md - Next phase planning document

Modified:
- ritz/parser.py - @external decorator support
- ritz/llvm_codegen_signals.py - External function handling
- ritz/syntax.py - Added external/bridge decorators, H4, Pre
- examples/runtime/ritz-runtime.js - Debug mode, stats updates
- examples/runtime/styles.css - Merged debug styles
- examples/template.html - Debug mode enabled by default
- examples/highfive/HighFive.html - Simplified using template

## Known Issues (Addressed in PHASE4_CLEANUP.md)

1. Debug console shifts page RIGHT (should shift LEFT)
2. Toggle implementation un-Ritzy (calls JS to manipulate DOM)
   - Should use reactive class binding + @event decorator
   - Needs event object bridge for keyboard events
3. Missing test infrastructure

## Next Steps (Phase 4)

See PHASE4_CLEANUP.md for detailed plan:
- Implement reactive class binding (signal-driven classes)
- Add @event decorator for declarative event registration
- Build event object bridge (access event.key, event.ctrlKey)
- Create Python test mode (execute without browser)
- Add comprehensive test suite
- Update all examples to use new patterns

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

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

examples/runtime/styles.css

@@ -98,3 +98,142 @@ ul {
 li {
     margin: 8px 0;
 }
+
+/* ========================================
+   Ritz Debug Console Styles
+   ======================================== */
+
+.ritz-debug {
+    position: fixed;
+    top: 0;
+    right: 0;
+    width: 400px;
+    height: 100vh;
+    background: #1e1e1e;
+    color: #d4d4d4;
+    font-family: 'Consolas', 'Monaco', monospace;
+    font-size: 13px;
+    box-shadow: -2px 0 10px rgba(0,0,0,0.3);
+    transition: transform 0.3s ease;
+    z-index: 9999;
+    overflow-y: auto;
+}
+
+.ritz-debug.hidden {
+    transform: translateX(100%);
+}
+
+.ritz-debug .debug-header {
+    background: #252525;
+    padding: 15px;
+    border-bottom: 1px solid #3e3e3e;
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+}
+
+.ritz-debug .debug-header h3 {
+    margin: 0;
+    font-size: 16px;
+    font-weight: 600;
+    color: #d4d4d4;
+}
+
+.ritz-debug .debug-header button {
+    background: none;
+    border: none;
+    color: #d4d4d4;
+    font-size: 24px;
+    cursor: pointer;
+    padding: 0;
+    width: 30px;
+    height: 30px;
+    line-height: 1;
+    transform: none;
+    box-shadow: none;
+}
+
+.ritz-debug .debug-header button:hover {
+    color: #fff;
+    background: none;
+    transform: none;
+    box-shadow: none;
+}
+
+#ritz-debug .status {
+    position: static !important;
+    margin: 15px;
+    padding: 10px;
+    background: #2d2d30;
+    border: none !important;
+    border-left: 3px solid #007acc !important;
+    border-radius: 3px;
+    font-size: 12px;
+    box-shadow: none !important;
+}
+
+#ritz-debug .status.success {
+    background: #2d2d30 !important;
+    border-left-color: #4ec9b0 !important;
+    color: #4ec9b0 !important;
+}
+
+#ritz-debug .status.error {
+    background: #2d2d30 !important;
+    border-left-color: #f48771 !important;
+    color: #f48771 !important;
+}
+
+.debug-content {
+    padding: 15px;
+}
+
+.debug-section {
+    margin-bottom: 20px;
+}
+
+.debug-section h4 {
+    margin: 0 0 10px 0;
+    font-size: 12px;
+    text-transform: uppercase;
+    color: #858585;
+    font-weight: 600;
+}
+
+.debug-section pre {
+    background: #252525;
+    padding: 10px;
+    border-radius: 3px;
+    overflow-x: auto;
+    margin: 0;
+    font-size: 11px;
+    line-height: 1.6;
+    color: #d4d4d4;
+}
+
+.debug-toggle-hint {
+    position: fixed;
+    bottom: 20px;
+    right: 20px;
+    background: #252525;
+    color: #d4d4d4;
+    padding: 8px 12px;
+    border-radius: 4px;
+    font-family: 'Consolas', 'Monaco', monospace;
+    font-size: 11px;
+    box-shadow: 0 2px 8px rgba(0,0,0,0.3);
+    opacity: 0;
+    animation: fadeInOut 3s ease-in-out;
+    pointer-events: none;
+    z-index: 10000;
+}
+
+@keyframes fadeInOut {
+    0%, 100% { opacity: 0; }
+    10%, 90% { opacity: 1; }
+}
+
+/* Adjust app container when debug is open */
+body.debug-open #app {
+    margin-right: 420px;
+}