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/bytecode.sx

@@ -93,6 +93,24 @@
 (define OP_STR_JOIN       145)  ;; (join sep list)
 (define OP_SERIALIZE      146)  ;; serialize TOS to SX string
 
+;; Inline primitives (hot path — no hashtable lookup)
+(define OP_ADD            160)  ;; TOS-1 + TOS → push
+(define OP_SUB            161)  ;; TOS-1 - TOS → push
+(define OP_MUL            162)  ;; TOS-1 * TOS → push
+(define OP_DIV            163)  ;; TOS-1 / TOS → push
+(define OP_EQ             164)  ;; TOS-1 = TOS → push bool
+(define OP_LT             165)  ;; TOS-1 < TOS → push bool
+(define OP_GT             166)  ;; TOS-1 > TOS → push bool
+(define OP_NOT            167)  ;; !TOS → push bool
+(define OP_LEN            168)  ;; len(TOS) → push number
+(define OP_FIRST          169)  ;; first(TOS) → push
+(define OP_REST           170)  ;; rest(TOS) → push list
+(define OP_NTH            171)  ;; nth(TOS-1, TOS) → push
+(define OP_CONS           172)  ;; cons(TOS-1, TOS) → push list
+(define OP_NEG            173)  ;; negate TOS → push number
+(define OP_INC            174)  ;; TOS + 1 → push
+(define OP_DEC            175)  ;; TOS - 1 → push
+
 ;; Aser specialization (optional, 224-239 reserved)
 (define OP_ASER_TAG       224)  ;; u16 tag_name_idx — serialize HTML tag
 (define OP_ASER_FRAG      225)  ;; u8 child_count — serialize fragment