checker: a function named like a built-in a call always takes is refused at its declaration - function words(st, k) compiled and every call became words(n) with a pointer for n (invalid IR, far from the cause); the table is selfhost/check/check_builtins.ludic, emit_call's built-ins no find_fn guard lets a declared function take, and ludic-dev syntax --check holds it to emit_call both ways; every other built-in (buffer, floats, double, ...) yields to a declared function in the checker as it already did in codegen. string_temps' keep and cross_heap_test's keep renamed (hand, keep_cell); rejected/builtin_call_name and functions/builtin_yield with their cases (added, not run); reseeded, bootstrap-cfree fixpoint holds

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

examples/functions/builtin_yield.ludic

@@ -0,0 +1,11 @@
+# builtin_yield.ludic - a built-in that a declared function may take (buffer, floats, double, ...)
+# yields to it in the checker as in codegen: this `buffer` is the program's own, returning text.
+#
+# Prints:  ab 4
+program BuiltinYield {
+  function buffer(a: string, b: string) -> string { return a + b }
+  entry {
+    let s = buffer("a", "b")
+    print(`{s} {len(floats(4))}`)
+  }
+}