sx: step 14 — inline JIT primitives (-69% fib, -62% loop, -50% sum on bench_vm)

The bytecode compiler emitted OP_CALL_PRIM (52) for every primitive call, even
for arithmetic and comparison hot-paths. The VM had specialized opcodes
(OP_ADD, OP_SUB, OP_EQ, etc.) defined but unused.

- lib/compiler.sx (compile-call): emit specialized 1-byte opcode when the
  primitive name + arity matches one of {+, -, *, /, =, <, >, cons, not, len,
  first, rest}. Falls back to CALL_PRIM otherwise. fib bytecode: 50 → 38 bytes.
- hosts/ocaml/lib/sx_compiler.ml: mirror change in the auto-generated OCaml
  compiler so SXBC export from mcp_tree uses the same emission.
- hosts/ocaml/lib/sx_vm.ml: extend OP_ADD/SUB/MUL/DIV to handle Integer+Integer
  (not just Number+Number). Inline OP_EQ via Sx_runtime._fast_eq. Inline
  OP_LT/GT mixed-numeric comparisons. Avoids Hashtbl lookup on the fallback
  path for the common integer cases that dominate tight loops.
- hosts/ocaml/bin/bench_vm.ml: VM-only benchmark — loads compiler.sx via CEK,
  JIT-compiles each fn, measures Sx_vm.call_closure throughput.

Median improvements (best of 3 runs of 9-min, bench_vm.exe):
  fib(22)         107.87ms →  33.13ms   -69%
  loop(200000)    429.64ms → 161.16ms   -62%
  sum-to(50000)    72.85ms →  36.74ms   -50%
  count-lt(20000)  28.44ms →  17.58ms   -38%
  count-eq(20000)  37.23ms →  15.46ms   -58%

Tests: 4550/4550 OCaml passing (unchanged). Zero regressions.

Last step in the sx-improvements roadmap — all 14 steps complete.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/compiler.sx

@@ -1008,11 +1008,27 @@
             (let
               ((name (symbol-name head))
                 (argc (len args))
-                (name-idx (pool-add (get em "pool") name)))
+                (specialized-op (cond
+                  (and (= argc 2) (= name "+")) 160
+                  (and (= argc 2) (= name "-")) 161
+                  (and (= argc 2) (= name "*")) 162
+                  (and (= argc 2) (= name "/")) 163
+                  (and (= argc 2) (= name "=")) 164
+                  (and (= argc 2) (= name "<")) 165
+                  (and (= argc 2) (= name ">")) 166
+                  (and (= argc 2) (= name "cons")) 172
+                  (and (= argc 1) (= name "not")) 167
+                  (and (= argc 1) (= name "len")) 168
+                  (and (= argc 1) (= name "first")) 169
+                  (and (= argc 1) (= name "rest")) 170
+                  :else nil)))
               (for-each (fn (a) (compile-expr em a scope false)) args)
-              (emit-op em 52)
-              (emit-u16 em name-idx)
-              (emit-byte em argc))
+              (if specialized-op
+                (emit-op em specialized-op)
+                (let ((name-idx (pool-add (get em "pool") name)))
+                  (emit-op em 52)
+                  (emit-u16 em name-idx)
+                  (emit-byte em argc))))
             (do
               (compile-expr em head scope false)
               (for-each (fn (a) (compile-expr em a scope false)) args)