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>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-24T19:19:01Z";
+  var SX_VERSION = "2026-03-24T20:05:12Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }