cgen: jolt-IR -> C for numeric-leaf fns (jolt-ihdp) (#145)

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.

Co-authored-by: Yogthos <yogthos@gmail.com>

spike/native/dump-ir.janet

@@ -0,0 +1,15 @@
+# Dump the analyzed IR (not emitted Janet) for count-point, so the C emitter can
+# be written against the real node shapes. jolt-ihdp.
+(import ../../src/jolt/api :as api)
+(import ../../src/jolt/backend :as backend)
+(import ../../src/jolt/reader :as reader)
+
+(def ctx (api/init-cached {:compile? true}))
+(put (ctx :env) :direct-linking? true)
+(put (ctx :env) :inline? true)
+(api/eval-string ctx "(ns mandelbrot)")
+
+(def src "(defn count-point [cr ci cap] (loop [i 0 zr 0.0 zi 0.0] (if (or (>= i cap) (> (+ (* zr zr) (* zi zi)) 4.0)) i (recur (inc i) (+ (- (* zr zr) (* zi zi)) cr) (+ (* 2.0 (* zr zi)) ci)))))")
+
+(def ir (backend/analyze-form ctx (reader/parse-string src)))
+(printf "%P" ir)