feat(camera): #78 deterministic Camera.zoom (Q16.16 render-time zoom)

The #78 investigation rejected hardware f32/f64 for the coordinate types
(they would desync lockstep/replay/save) and identified camera zoom as the
one genuinely-missing render feature. Ship it: Camera.zoom(scale) scales the
whole view about the screen centre by a Q16.16 factor, threaded through the
same two framebuffer chokepoints (rt_put_px/rt_fill_rect) that carry the
camera offset, so it composes with Camera.set/follow/shake. Gated by an
internal rt_cam_zoomed flag so a game that never zooms renders byte-for-byte
identically (golden renders unchanged); Camera.zoom(1.0) turns it back off.
The world coordinate types stay integer px + Q16.16 velocity, so it's a pure
render-time transform and itself deterministic.

Example examples/library/camera_zoom.ludic (pixel-readback verified),
docs page, RFC updated (docs/RFC-POSITION-TYPES.md). Full suite 105/0,
fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/emit_call.ludic

@@ -183,6 +183,7 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "set")    { bare = "camera";        push(labels, "x"); push(labels, "y") }
     if (meth == "follow") { bare = "camera_follow"; push(labels, "x"); push(labels, "y"); push(labels, "lerp") }
     if (meth == "shake")  { bare = "camera_shake";  push(labels, "amount") }
+    if (meth == "zoom")   { bare = "camera_zoom";   push(labels, "scale") }   # #78 deterministic Q16.16 zoom
   }
   if (ns == "Map") {
     if (meth == "size") { bare = "map_size"; push(labels, "width"); push(labels, "height") }