Roblox backend: hidden TextBox proxy for text input

A single off-screen TextBox holds focus while an imgui text field is active, so the OS
delivers typing (any layout/unicode), paste and copy/cut. Its text mirrors imgui selection
(fully selected) so Ctrl+C/X copy exactly that; deletions apply as an imgui Backspace;
Ctrl is released around pasted characters. Focus is captured after the click is released,
kept on clicks inside imgui, ends on clicks outside. KeyCode text kept as fallback.
SetTextProxyDebug for logging.

tools/test/roblox_mock.luau

@@ -60,6 +60,7 @@ local InstMethods = {}
 local InstMT = {}
 InstMT.__index = function(t, k)
     if k == "Parent" then return rawget(t, "_parent") end
+    local pv = rawget(t, "_props")[k]; if pv ~= nil then return pv end
     local m = InstMethods[k]; if m then return m end
     for _, c in ipairs(rawget(t, "_children")) do if c.Name == k then return c end end
     return nil
@@ -71,13 +72,18 @@ InstMT.__newindex = function(t, k, v)
         rawset(t, "_parent", v)
         if v then table.insert(rawget(v, "_children"), t) end
     else
-        rawset(t, k, v)
+        local props = rawget(t, "_props")
+        local old = props[k]
+        props[k] = v
+        local sigs = rawget(t, "_propsigs")
+        if sigs and sigs[k] and old ~= v then sigs[k]:Fire() end
     end
 end
 local function newInst(class, name, parent)
-    local o = setmetatable({ ClassName = class, Name = name or class, _children = {} }, InstMT)
+    local o = setmetatable({ _children = {}, _props = { ClassName = class, Name = name or class } }, InstMT)
     if class == "ScreenGui" or class == "Frame" or class == "ImageLabel" then o.AbsoluteSize = v2(SCREEN_W, SCREEN_H) end
     o.Visible = true
+    if class == "TextBox" then rawget(o, "_props").FocusLost = Signal() end
     if class == "BindableEvent" then local sig = Signal(); rawset(o, "Event", sig); rawset(o, "Fire", function(_, ...) sig:Fire(...) end) end
     if parent then o.Parent = parent end
     return o
@@ -95,6 +101,14 @@ function InstMethods:IsA(class)
     return self.ClassName == class
 end
 function InstMethods:Destroy() self.Parent = nil end
+local focused_box = nil
+function InstMethods:GetPropertyChangedSignal(prop)
+    local sigs = rawget(self, "_propsigs") or {}; rawset(self, "_propsigs", sigs)
+    sigs[prop] = sigs[prop] or Signal(); return sigs[prop]
+end
+function InstMethods:CaptureFocus() focused_box = self end
+function InstMethods:_loseFocus() if focused_box == self then focused_box = nil; local fl = rawget(self, "_props").FocusLost; if fl then fl:Fire(false) end end end
+function InstMethods:ReleaseFocus() self:_loseFocus() end
 
 -- Services
 local mouse = v2(0, 0)
@@ -103,6 +117,7 @@ local UIS = {
     InputBegan = Signal(), InputEnded = Signal(), InputChanged = Signal(), WindowFocused = Signal(), WindowFocusReleased = Signal(),
 }
 function UIS:GetMouseLocation() return mouse end
+function UIS:GetFocusedTextBox() return focused_box end
 function UIS:IsKeyDown(kc) return keys_down[kc] == true end
 
 local RunService = { Heartbeat = Signal(), Stepped = Signal(), PreAnimation = Signal(), RenderStepped = Signal() }