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.
327c8af9b7