Add inline VM opcodes for hot primitives (OP_ADD through OP_DEC)

16 new opcodes (160-175) bypass the CALL_PRIM hashtable lookup for
the most frequently called primitives:

  Arithmetic: OP_ADD, OP_SUB, OP_MUL, OP_DIV, OP_INC, OP_DEC, OP_NEG
  Comparison: OP_EQ, OP_LT, OP_GT, OP_NOT
  Collection: OP_LEN, OP_FIRST, OP_REST, OP_NTH, OP_CONS

The compiler (compiler.sx) recognizes these names at compile time and
emits the inline opcode instead of CALL_PRIM. The opcode is self-
contained — no constant pool index, no argc byte. Each primitive is
a single byte in the bytecode stream.

Implementation in all three VMs:
- OCaml (sx_vm.ml): direct pattern match, no allocation
- SX spec (vm.sx): delegates to existing primitives
- JS (transpiled): same as SX spec

66 new tests in spec/tests/vm-inline.sx covering arithmetic, comparison,
collection ops, composition, and edge cases.

Tests: 1314 JS (full), 1114 OCaml, 32 Playwright

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/vm.sx

@@ -531,6 +531,49 @@
                 (name (nth consts idx)))
             (dict-set! (get vm "globals") name (vm-peek vm)))
 
+        ;; ---- Inline primitives ----
+        (= op 160) ;; OP_ADD
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (+ a b)))
+        (= op 161) ;; OP_SUB
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (- a b)))
+        (= op 162) ;; OP_MUL
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (* a b)))
+        (= op 163) ;; OP_DIV
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (/ a b)))
+        (= op 164) ;; OP_EQ
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (= a b)))
+        (= op 165) ;; OP_LT
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (< a b)))
+        (= op 166) ;; OP_GT
+          (let ((b (vm-pop vm)) (a (vm-pop vm)))
+            (vm-push vm (> a b)))
+        (= op 167) ;; OP_NOT
+          (vm-push vm (not (vm-pop vm)))
+        (= op 168) ;; OP_LEN
+          (vm-push vm (len (vm-pop vm)))
+        (= op 169) ;; OP_FIRST
+          (vm-push vm (first (vm-pop vm)))
+        (= op 170) ;; OP_REST
+          (vm-push vm (rest (vm-pop vm)))
+        (= op 171) ;; OP_NTH
+          (let ((n (vm-pop vm)) (coll (vm-pop vm)))
+            (vm-push vm (nth coll n)))
+        (= op 172) ;; OP_CONS
+          (let ((coll (vm-pop vm)) (x (vm-pop vm)))
+            (vm-push vm (cons x coll)))
+        (= op 173) ;; OP_NEG
+          (vm-push vm (- 0 (vm-pop vm)))
+        (= op 174) ;; OP_INC
+          (vm-push vm (inc (vm-pop vm)))
+        (= op 175) ;; OP_DEC
+          (vm-push vm (dec (vm-pop vm)))
+
         :else
           (error (str "VM: unknown opcode " op))))))