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/UISystem.cpp

@@ -0,0 +1,100 @@
+#include "zeroerr/assert.h"
+#include "zeroerr/unittest.h"
+
+#include "ui/UIHost.h"
+#include "ui/UISystem.h"
+#include "ui/Widget.h"
+
+#include <imgui.h>
+
+using namespace eve::ui;
+
+TEST_CASE("WidgetDesc.composeNestedTree") {
+    int clicks = 0;
+    UIHost *h = UIHost::createHost("nest");
+    h->setTree(window(
+        "Inventory",
+        {
+            text("Hello", "label"),
+            group({
+                button("A", "a"),
+                sameLine(),
+                button("B", "b", [&]() { ++clicks; }),
+            }),
+        },
+        "root"));
+
+    auto t = h->tree();
+    REQUIRE_EQ(t->root, 0);
+    CHECK_EQ(int(t->nodes[0].type), int(NodeType::Window));
+    REQUIRE_GE(t->nodes.size(), 5u);
+
+    UINode *b = h->findById("b");
+    REQUIRE(b != nullptr);
+    REQUIRE_NE(b->handlerClick, 0u);
+
+    UIEvent ev;
+    ev.host = h;
+    ev.hostName = "nest";
+    ev.nodeId = "b";
+    ev.kind = "click";
+    ev.handlerIndex = b->handlerClick;
+    UISystem::pendingEvents().push_back(ev);
+    UISystem::dispatchEvents();
+    CHECK_EQ(clicks, 1);
+    CHECK(UISystem::consumeClickFor("nest") == "b");
+}
+
+TEST_CASE("UIHost.setSimplePanel.buildsRetainedTree") {
+    UIHost *h = UIHost::createHost("simple");
+    h->setSimplePanel("Inventory", "Hello", "Use");
+    auto t = h->tree();
+    REQUIRE_EQ(t->root, 0);
+    REQUIRE_EQ(t->nodes.size(), 3u);
+    CHECK(t->nodes[1].id == "label");
+    CHECK(t->nodes[2].id == "btn");
+}
+
+TEST_CASE("UIHost.setPropsAndClickHandler") {
+    UIHost *h = UIHost::createHost("props");
+    h->setTree(window("W", {text("before", "label"), button("Go", "btn")}));
+    h->setTextById("label", "after");
+    CHECK(h->findById("label")->text == "after");
+    h->setVisibleById("btn", false);
+    CHECK(!h->findById("btn")->visible);
+
+    int clicks = 0;
+    CHECK(h->setClickHandler("btn", [&]() { ++clicks; }));
+
+    UIEvent ev;
+    ev.host = h;
+    ev.hostName = "props";
+    ev.nodeId = "btn";
+    ev.kind = "click";
+    ev.handlerIndex = h->findById("btn")->handlerClick;
+    UISystem::pendingEvents().push_back(ev);
+    UISystem::dispatchEvents();
+    CHECK_EQ(clicks, 1);
+}
+
+TEST_CASE("UISystem.render.headlessImGuiWalk") {
+    UIHost *h = UIHost::createHost("walk");
+    h->setTree(window("Panel", {text("Label", "label"), button("Btn", "btn")}));
+
+    IMGUI_CHECKVERSION();
+    ImGui::CreateContext();
+    ImGuiIO &io = ImGui::GetIO();
+    io.DisplaySize = ImVec2(800.f, 600.f);
+    io.IniFilename = nullptr;
+    unsigned char *pixels = nullptr;
+    int w = 0, htex = 0;
+    io.Fonts->GetTexDataAsRGBA32(&pixels, &w, &htex);
+    (void)pixels;
+
+    ImGui::NewFrame();
+    UISystem::render();
+    ImGui::EndFrame();
+    ImGui::DestroyContext();
+
+    CHECK(!h->tree()->dirty);
+}