platform: protocol skeleton (types + api, no backend yet)

First commit of the Phase 8 platform abstraction (see current/PLATFORM_PLAN.md):

- `library/modules/platform/types.sx` — `FrameContext` (viewport_w/h,
  pixel_w/h, dpi_scale, delta_time) and `KeyboardState` (visible, height
  + `zero()`). `EdgeInsets`/`Point`/`Size` and `Event` are reused from
  `modules/ui/types.sx` / `modules/ui/events.sx`.
- `library/modules/platform/api.sx` — `Platform :: protocol { init,
  run_frame_loop, poll_events, begin_frame, end_frame, safe_insets,
  keyboard, show_keyboard, hide_keyboard, shutdown }`. Protocol bodies
  omit `self` (matches the `View`/`Allocator` convention).
- `run_frame_loop` takes `Closure()` so backends own the run loop:
  SDL drives a `while !quit`, UIKit hands it to a `CADisplayLink` tick,
  Emscripten hands it to `emscripten_set_main_loop`.

No backend yet. Regression suite still 50/50; game build still green.

library/modules/platform/api.sx

@@ -0,0 +1,22 @@
+#import "modules/std.sx";
+#import "modules/ui/types.sx";
+#import "modules/ui/events.sx";
+#import "modules/platform/types.sx";
+
+Platform :: protocol {
+    init :: (title: [:0]u8, w: s32, h: s32) -> bool;
+
+    run_frame_loop :: (frame_fn: Closure());
+
+    poll_events :: () -> []Event;
+
+    begin_frame :: () -> FrameContext;
+    end_frame :: ();
+
+    safe_insets :: () -> EdgeInsets;
+    keyboard :: () -> KeyboardState;
+    show_keyboard :: ();
+    hide_keyboard :: ();
+
+    shutdown :: ();
+}

library/modules/platform/types.sx

@@ -0,0 +1,19 @@
+#import "modules/std.sx";
+#import "modules/ui/types.sx";
+#import "modules/ui/events.sx";
+
+FrameContext :: struct {
+    viewport_w: f32;
+    viewport_h: f32;
+    pixel_w: s32;
+    pixel_h: s32;
+    dpi_scale: f32;
+    delta_time: f32;
+}
+
+KeyboardState :: struct {
+    visible: bool;
+    height: f32;
+
+    zero :: () -> KeyboardState => .{ visible = false, height = 0.0 };
+}