feat(rendering): add Screen.camera/clip/blend_mode/oval + Camera.* + Screen.pixel (#23)

Completes the transform/state-based rendering #23 tracked as blocked on new
renderer state. All of it threads through the two framebuffer chokepoints every
draw primitive already funnels through (rt_put_px / rt_fill_rect), so one place
gives the whole draw API a camera, a clip rect, and a blend mode. Defaults are
neutral — camera (0,0), clip = full screen, blend = replace — so every existing
golden render is byte-identical (the 60+ render tests still pass unchanged).

New renderer state (runtime/native/core.ludic):
  - Screen.camera(x, y) / Camera.set(x, y)   world-space draw offset; a world
                                             point draws at (wx-x, wy-y). Moves
                                             everything — reset to (0,0) for a HUD.
  - Camera.follow(x, y, lerp)                ease the offset toward centring a
                                             target (fixed lerp 0..1)
  - Camera.shake(amount)                     +/- amount jitter from the seeded RNG
                                             (replay shakes identically); 0 clears
  - Screen.clip(x,y,w,h) / clip_reset()      screen-space clip rectangle
  - Screen.blend_mode(m)                     0 = replace, 1 = additive (clamped)

New primitives:
  - Screen.oval(x, y, rx, ry, color)         axis-aligned ellipse outline (midpoint)
  - Screen.measure_text(text) -> int         advance width in the 5x7 font
  - Screen.pixel(x, y) -> int                read a framebuffer pixel (0x00RRGGBB)

Everything stays integer and deterministic (the camera, shake, and blend all
reproduce exactly under identical inputs), so headless renders remain diffable.
Camera.follow interpolates in the fixed domain (fixed*fixed then floor) to avoid
the int*fixed coercion trap.

Screen.pixel makes the whole surface testable by reading rendered pixels back:
examples/library/render.ludic asserts 18 cases — pixel round-trip, camera and
Camera.set/follow offsets, clip in/out + reset, additive blend with 255 clamp,
oval extremes vs hollow centre, and text measurement — all verified against the
actual framebuffer, not just that the call compiled. Wired into x test (now 65
passed). Docs: 7 new Screen pages + a Camera section with 3 pages,
inventory/coverage green. Seed reseeded; the C-free bootstrap fixpoint holds.

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

selfhost/backend/emit_call.ludic

@@ -127,6 +127,21 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "fill_triangle")  { bare = "fill_triangle"; push(labels, "x1"); push(labels, "y1"); push(labels, "x2"); push(labels, "y2"); push(labels, "x3"); push(labels, "y3"); push(labels, "color") }
     if (meth == "sprite")         { bare = "draw_sprite"; push(labels, "id"); push(labels, "x"); push(labels, "y") }
     if (meth == "sprite_scaled")  { bare = "draw_sprite_scaled"; push(labels, "id"); push(labels, "x"); push(labels, "y"); push(labels, "scale") }
+    if (meth == "oval")           { bare = "oval";      push(labels, "x"); push(labels, "y"); push(labels, "rx"); push(labels, "ry"); push(labels, "color") }
+    if (meth == "camera")         { bare = "camera";    push(labels, "x"); push(labels, "y") }
+    if (meth == "clip")           { bare = "clip";      push(labels, "x"); push(labels, "y"); push(labels, "width"); push(labels, "height") }
+    if (meth == "clip_reset")     { bare = "clip_reset" }
+    if (meth == "blend_mode")     { bare = "blend_mode"; push(labels, "mode") }
+    if (meth == "measure_text")   { bare = "measure_text"; push(labels, "text") }
+    if (meth == "pixel")          { bare = "get_px";    push(labels, "x"); push(labels, "y") }
+  }
+  # Camera.* — the world-space camera: a draw offset threaded through the render
+  # path (runtime/native/core.ludic). set/follow move it; shake jitters it from
+  # the seeded RNG, so a replay shakes identically.
+  if (ns == "Camera") {
+    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 (ns == "Map") {
     if (meth == "size") { bare = "map_size"; push(labels, "width"); push(labels, "height") }