feat: implement file watching functionality in the filesystem module

- Added methods for file watching, including `watch`, `unwatch`, `unwatchAll`, `getWatchCount`, `pollWatch`, `getLastWatchPath`, and `getLastWatchRealPath`.
- Enhanced the `Filesystem` class to support monitoring file changes, allowing for real-time updates.
- Updated the `physfs` implementation to handle file watch targets and events.
- Improved documentation and added unit tests to validate the new file watching features.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/UIOverlaySmoke.cpp

@@ -0,0 +1,101 @@
+#include "zeroerr/assert.h"
+#include "zeroerr/unittest.h"
+
+#include "graphics/Graphics.h"
+#include "graphics/RenderSystem.h"
+#include "ui/UI.h"
+#include "ui/UISystem.h"
+#include "ui/Widget.h"
+#include "window/Window.h"
+
+#include <SDL2/SDL.h>
+
+#include <cmath>
+
+using namespace eve::graphics;
+using namespace eve::ui;
+
+/**
+ * Real window + Vulkan: world sprites (RenderSystem) and ImGui UI overlay in one present.
+ * Frame order: UI NewFrame/widgets → RenderSystem (draws + present with overlay) → dispatch.
+ */
+TEST_CASE("UI.smoke.RenderSystemSameFrameVulkanOverlay") {
+    auto *win = eve::window::Window::create();
+    auto *gfx = Graphics::create();
+    auto *ui = UI::create();
+    REQUIRE(win != nullptr);
+    REQUIRE(gfx != nullptr);
+    REQUIRE(ui != nullptr);
+
+    win->setGraphics(gfx);
+    eve::window::WindowSettings s;
+    s.width = 640;
+    s.height = 480;
+    s.centered = true;
+    REQUIRE(win->setWindowSettings(s));
+    REQUIRE_GT(gfx->getWidth(), 0);
+    REQUIRE_GT(gfx->getPixelWidth(), 0);
+
+    REQUIRE(ui->initBackend());
+    CHECK(ui->isBackendReady());
+    const bool overlayHooked = gfx->getPresentOverlay() != nullptr && gfx->getPresentOverlayUser() != nullptr;
+    CHECK(overlayHooked);
+
+    gfx->setBackgroundColor(Color(0.10f, 0.12f, 0.18f, 1.0f));
+    gfx->setScreenReadbackEnabled(true);
+
+    auto *cam = Camera2D::createCamera();
+    cam->data()->r = 0.10f;
+    cam->data()->g = 0.12f;
+    cam->data()->b = 0.18f;
+
+    auto *solid = Renderable2D::create();
+    solid->transform()->x = 80;
+    solid->transform()->y = 100;
+    solid->sprite()->width = 160;
+    solid->sprite()->height = 100;
+    solid->sprite()->r = 0.2f;
+    solid->sprite()->g = 0.85f;
+    solid->sprite()->b = 0.35f;
+    solid->sprite()->visible = true;
+
+    ui->mountAs("smoke",
+                window("Overlay",
+                       {
+                           text("UI+RenderSystem", "label"),
+                           button("Ping", "ping"),
+                       },
+                       "root"));
+
+    int framesOk = 0;
+    for (int frame = 0; frame < 30; ++frame) {
+        solid->transform()->x = 80.0f + float(frame) * 2.0f;
+
+        // ImGui frame must be built before present (RenderSystem ends with present).
+        ui->beginFrameAndRender();
+        RenderSystem::render(*gfx);
+        ui->dispatchEvents();
+
+        SDL_Event e;
+        while (SDL_PollEvent(&e)) {
+            ui->processEvent(&e);
+            if (e.type == SDL_QUIT) break;
+        }
+
+        // After present+readback, swapchain content should be readable.
+        Color p = gfx->getPixel(gfx->getPixelWidth() / 2, gfx->getPixelHeight() / 2);
+        CHECK(std::isfinite(p.r));
+        CHECK(std::isfinite(p.g));
+        CHECK(std::isfinite(p.b));
+        ++framesOk;
+
+        SDL_Delay(8);
+    }
+
+    CHECK_EQ(framesOk, 30);
+    CHECK(UISystem::findHost("smoke") != nullptr);
+    CHECK(ui->current()->findById("label") != nullptr);
+
+    gfx->setScreenReadbackEnabled(false);
+    win->close();
+}