First slice of the native-codegen tier. A new standalone module, src/jolt/
cgen.janet, that translates a numeric-leaf fn (numeric in/out, body uses only
native-op arithmetic + loop/recur/if/let/do) to a Janet native C module: params
unboxed to C doubles at entry, loop/recur lowered to a while loop, reboxed at
return. compile-fn runs cc and loads the .so via the native builtin, returning a
cfunction; it returns nil for non-candidates or when the toolchain is absent.
count-point compiles and matches the bytecode fn across the mandelbrot grid
(test/integration/cgen-test.janet, which skips the behavioral leg where cc/janet.h
are missing). Nothing wires this into the default compile path yet — detecting
hot fns and installing the C version onto the var cell is the next step.
See docs/foundational-runtime-lever1-native-codegen.md for the ceiling
(native-C ~18-22x faster than bytecode, edges out JVM) and the leaf-first rule.
