feat(compile): Phase 2 — native ops + direct calls (fib30 50s -> 0.076s, ~660x)

Two changes unlock native Janet speed in compile mode:

- Hot numeric primitives (+ - * < > <= >=) emit as native Janet SYMBOLS rather
  than the variadic core fns, so Janet's compiler uses its arithmetic/compare
  opcodes. = / not= / quot / rem / mod / division stay as core fns (their
  semantics differ from Janet's). Trade-off: the strict non-number checks are
  relaxed under compilation (documented perf-mode divergence).
- emit-invoke emits a DIRECT call (f arg...) when the callee is a function
  reference (core/local/symbol/fn), instead of wrapping every call in jolt-call.
  jolt-call is kept only for keyword/collection literals in call position
  ((:k m), ({:a 1} :a)) so IFn dispatch still works.

compiled fib(30): 3.4s -> 0.076s (native ceiling), faster than jank's 0.8s.
Updated compiler-test string assertions (core-+ -> +); compile-mode-test gains
native-op + IFn-dispatch cases; README documents compile mode. jpm test green.

test/unit/compiler-test.janet

@@ -62,7 +62,7 @@
 # ============================================================
 (print "6: let...")
 (assert (= "(let [x 1] (core-inc x))" (compile-str "(let* [x 1] (inc x))")) "let single binding")
-(assert (= "(let [x 1 y 2] (core-+ x y))" (compile-str "(let* [x 1 y 2] (+ x y))")) "let two bindings")
+(assert (= "(let [x 1 y 2] (+ x y))" (compile-str "(let* [x 1 y 2] (+ x y))")) "let two bindings")
 (assert (= "(let [x (core-inc 1)] (core-inc x))" (compile-str "(let* [x (inc 1)] (inc x))")) "let with fn in binding")
 (print "  passed")
 
@@ -71,8 +71,8 @@
 # ============================================================
 (print "7: invoke...")
 (assert (= "(core-inc 1)" (compile-str "(inc 1)")) "inc call")
-(assert (= "(core-+ 1 2)" (compile-str "(+ 1 2)")) "+ call")
-(assert (= "(core-+ (core-inc 1) 2)" (compile-str "(+ (inc 1) 2)")) "nested calls")
+(assert (= "(+ 1 2)" (compile-str "(+ 1 2)")) "+ call")
+(assert (= "(+ (core-inc 1) 2)" (compile-str "(+ (inc 1) 2)")) "nested calls")
 (assert (= "(core-map core-inc (core-vec 1 2 3))"
            (compile-str "(map inc (vec 1 2 3))")) "multi-arg call")
 (print "  passed")