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>

docs/language/camera/camera-zoom.md

@@ -0,0 +1,31 @@
+---
+id: camera-zoom
+name: Camera.zoom
+category: camera
+kind: namespace-method
+tokens: Camera.zoom
+sig: Camera.zoom(scale)
+tip: Scale the whole view about the screen centre by a Q16.16 factor (1.0 = none).
+order: 4
+ns: Camera
+member: zoom
+---
+
+Sets a render-time zoom: every draw is scaled about the screen centre by <code>scale</code>, a Q16.16 <code>fixed</code> factor. <code>1.0</code> is no zoom (and turns the zoom path back off, restoring the byte-identical un-zoomed blit); <code>2.0</code> is 2x in; <code>0.5</code> is 2x out. It composes with <a href="camera-set"><code>Camera.set</code></a> / <a href="camera-follow"><code>Camera.follow</code></a> (which move the view) and <a href="camera-shake"><code>Camera.shake</code></a>.
+
+Zoom is a *render-time transform*, not a change to the world coordinate types — those stay integer pixels + Q16.16 velocity so lockstep, replay and <code>world_save</code> hold (see the position-types RFC, #78). Because it rides on fixed-point, the zoom is itself deterministic: the same <code>scale</code> produces the same pixels on every machine and headless.
+
+Parameters:
+- `scale` — the zoom factor (`fixed`); `1.0` = no zoom, `>1` zooms in, `<1` zooms out
+
+```ludic
+program Demo {
+  property Cam { z: fixed = 1.0 }
+  model View { Cam }
+  handler DrawWorld phase Render {
+    Camera.zoom(2.0)
+    Screen.fill_rectangle(150, 110, 20, 20, 0xffcc00)
+    Screen.show()
+  }
+}
+```