feat(lang): functions are values (L2), and render3d takes its scene as callbacks

fn(int, float) -> bool is a type, fn name is any top-level function's value, and a call through
a local, a global, a record field, a slice element, a parameter or a result of a function type
is an indirect call; two function types mix only when equal, a call checks its argument count,
and a value may be null (examples/functions/values.ludic). Job.parallel_for keeps its worker
check.

render3d's scene is registered rather than required by name: r3d_on_draw, r3d_on_casters and
r3d_on_stream_fill (hooks.ludic). The two rendering examples register theirs - and had defined
scene_draw_casters with no parameter while the renderer passed one, which nothing checked.
render3d declares numbers float itself; smooth.ludic is converted to floats and returns when
r3d_init fails instead of running on into a segfault. Noise.* check their argument count (a call
one short crashed the compiler). selfhost-build says why it failed. The migration tool reads a
declared float as evidence. Seed regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/functions/values.ludic

@@ -0,0 +1,31 @@
+# values.ludic — L2: a function is a value. `fn(int) -> int` is a type; `fn name` is any
+# top-level function's value; a call through a local, a global, a field or an element of a
+# function type is an indirect call. A registry can hold behaviour, and a package can take one.
+#
+# Running it prints:  42 10 25 49 81 1.5 1
+program FunctionValues {
+  numbers float
+  property Kind { name: string = "", use: fn(int) -> int = null }
+  function double(x: int) -> int { return x * 2 }
+  function square(x: int) -> int { return x * x }
+  function half(x: float) -> float { return x / 2.0 }
+  var op: fn(int) -> int = null
+  function run_with(f: fn(int) -> int, v: int) -> int { return f(v) }
+  function pass_on(f: fn(float) -> float) -> fn(float) -> float { return f }
+  entry {
+    op = fn double
+    print(op(21))
+    let ops = new []fn(int) -> int
+    push(ops, fn double)
+    push(ops, fn square)
+    for i in 0 .. len(ops) { print(ops[i](5)) }
+    let k = new Kind
+    k.name = "square"
+    k.use = fn square
+    print(k.use(7))
+    print(run_with(fn square, 9))
+    let h = pass_on(fn half)
+    print(h(3.0))
+    if op != null { print(1) }
+  }
+}