feat: Add comprehensive profiling system with F3 overlay

Add real-time performance profiling infrastructure to monitor frame times,
render performance, and identify bottlenecks.

Features:
- Profiler.h: ScopedTimer RAII helper for automatic timing measurements
- ProfilerOverlay: F3-togglable overlay displaying real-time metrics
- Detailed timing breakdowns: grid rendering, entity rendering, FOV,
  Python callbacks, and animation updates
- Per-frame counters: cells rendered, entities rendered, draw calls
- Performance color coding: green (<16ms), yellow (<33ms), red (>33ms)
- Benchmark suite: static grid and moving entities performance tests

Integration:
- GameEngine: Integrated profiler overlay with F3 toggle
- UIGrid: Added timing instrumentation for grid and entity rendering
- Metrics tracked in ProfilingMetrics struct with 60-frame averaging

Usage:
- Press F3 in-game to toggle profiler overlay
- Run benchmarks with tests/benchmark_*.py scripts
- ScopedTimer automatically measures code block execution time

This addresses issue #104 (Basic profiling/metrics).

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

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

src/Profiler.cpp

@@ -0,0 +1,61 @@
+#include "Profiler.h"
+#include <iostream>
+
+ProfilingLogger::ProfilingLogger()
+    : headers_written(false)
+{
+}
+
+ProfilingLogger::~ProfilingLogger() {
+    close();
+}
+
+bool ProfilingLogger::open(const std::string& filename, const std::vector<std::string>& columns) {
+    column_names = columns;
+    file.open(filename);
+
+    if (!file.is_open()) {
+        std::cerr << "Failed to open profiling log file: " << filename << std::endl;
+        return false;
+    }
+
+    // Write CSV header
+    for (size_t i = 0; i < columns.size(); ++i) {
+        file << columns[i];
+        if (i < columns.size() - 1) {
+            file << ",";
+        }
+    }
+    file << "\n";
+    file.flush();
+
+    headers_written = true;
+    return true;
+}
+
+void ProfilingLogger::writeRow(const std::vector<float>& values) {
+    if (!file.is_open()) {
+        return;
+    }
+
+    if (values.size() != column_names.size()) {
+        std::cerr << "ProfilingLogger: value count (" << values.size()
+                  << ") doesn't match column count (" << column_names.size() << ")" << std::endl;
+        return;
+    }
+
+    for (size_t i = 0; i < values.size(); ++i) {
+        file << values[i];
+        if (i < values.size() - 1) {
+            file << ",";
+        }
+    }
+    file << "\n";
+}
+
+void ProfilingLogger::close() {
+    if (file.is_open()) {
+        file.flush();
+        file.close();
+    }
+}