All 40 VM tests pass: map/filter/for-each + mutable closures fixed

Two fixes:

1. HO forms (map/filter/for-each/reduce): registered as Python
   primitives so compiler emits OP_CALL_PRIM (direct dispatch to
   OCaml primitive) instead of OP_CALL (which routed through CEK
   HO special forms and failed on NativeFn closure args).

2. Mutable closures: locals captured by closures now share an
   upvalue_cell. OP_LOCAL_GET/SET check frame.local_cells first —
   if the slot has a shared cell, read/write through it. OP_CLOSURE
   creates or reuses cells for is_local=1 captures. Both parent
   and closure see the same mutations.

   Frame type extended with local_cells hashtable for captured slots.

40/40 tests pass:
  - 12 compiler output tests
  - 18 VM execution tests (arithmetic, control flow, closures,
    nested let, higher-order, cond, string ops)
  - 10 auto-compile pattern tests (recursive, map, filter,
    for-each, mutable closures, multiple closures, type dispatch)

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

shared/sx/tests/test_vm_compile.py

@@ -35,6 +35,11 @@ PRIMITIVES['primitive?'] = lambda name: isinstance(name, str) and name in PRIMIT
 PRIMITIVES['has-key?'] = lambda *a: isinstance(a[0], dict) and str(a[1]) in a[0]
 PRIMITIVES['set-nth!'] = lambda *a: (a[0].__setitem__(int(a[1]), a[2]), NIL)[-1]
 PRIMITIVES['init'] = lambda *a: a[0][:-1] if isinstance(a[0], list) else a[0]
+
+# Register HO forms as primitives so compiler emits CALL_PRIM (direct dispatch)
+# instead of CALL (which routes through CEK HO special forms)
+for _ho_name in ['map', 'map-indexed', 'filter', 'reduce', 'for-each', 'some', 'every?']:
+    PRIMITIVES[_ho_name] = lambda *a: NIL  # placeholder — OCaml primitives handle actual work
 PRIMITIVES['make-symbol'] = lambda name: Symbol(name)
 PRIMITIVES['concat'] = lambda *a: (a[0] or []) + (a[1] or [])
 PRIMITIVES['slice'] = lambda *a: a[0][int(a[1]):int(a[2])] if len(a) == 3 else a[0][int(a[1]):]