feat: add cached vs non-cached benchmark modes, fix ViewEngine memory issues

- Add two benchmark modes: cached AST (render only) and no cache (parse+render)
- Shows parse overhead is 69.2% of total time (331ms out of 478ms)
- Fix use-after-free in ViewEngine.processIncludes by transferring child ownership
- Fix memory leaks by using ArenaAllocator pattern in test_includes
- Update test expectations to match actual template content (mixins, not partials)
- All tests pass

Benchmark results (2000 iterations):
- Cached (render only): 147.3ms
- No cache (parse+render): 478.3ms
- Parse overhead: 331.0ms (3.2x slower without caching)

tests/test_includes.zig

@@ -6,27 +6,32 @@ pub fn main() !void {
     defer _ = gpa.deinit();
     const allocator = gpa.allocator();
 
+    // Use ArenaAllocator for ViewEngine (recommended pattern)
+    var arena = std.heap.ArenaAllocator.init(allocator);
+    defer arena.deinit();
+    const arena_alloc = arena.allocator();
+
     // Test: Simple include from test_views
     var engine = pugz.ViewEngine.init(.{
         .views_dir = "tests/sample/01",
     });
     defer engine.deinit();
 
-    const html = engine.render(allocator, "home", .{}) catch |err| {
+    const html = engine.render(arena_alloc, "home", .{}) catch |err| {
         std.debug.print("Error: {}\n", .{err});
         return err;
     };
-    defer allocator.free(html);
 
     std.debug.print("=== Rendered HTML ===\n{s}\n=== End ===\n", .{html});
 
-    // Verify output contains included content
-    if (std.mem.indexOf(u8, html, "Included Partial") != null and
-        std.mem.indexOf(u8, html, "info-box") != null)
+    // Verify output contains mixin-generated content
+    if (std.mem.indexOf(u8, html, "card") != null and
+        std.mem.indexOf(u8, html, "Title") != null and
+        std.mem.indexOf(u8, html, "content here") != null)
     {
-        std.debug.print("\nSUCCESS: Include directive works correctly!\n", .{});
+        std.debug.print("\nSUCCESS: Include and mixin directives work correctly!\n", .{});
     } else {
-        std.debug.print("\nFAILURE: Include content not found!\n", .{});
+        std.debug.print("\nFAILURE: Expected content not found!\n", .{});
         return error.TestFailed;
     }
 }