VM upvalue support: closures capture variables from enclosing scopes

Compiler (compiler.sx):
- Function scopes marked is-function=true; let scopes share parent frame
- scope-resolve only creates upvalue captures at function boundaries
- Let scope locals use parent's slot numbering (same frame)
- OP_CLOSURE emits upvalue descriptors: (is_local, index) per capture

VM (sx_vm.ml):
- upvalue_cell type: shared mutable reference to captured value
- OP_UPVALUE_GET/SET: read/write from closure's upvalue array
- OP_CLOSURE: reads upvalue descriptors, creates cells from
  enclosing frame's locals (is_local=1) or upvalues (is_local=0)
- vm_closure carries live env_ref (not snapshot)
- vm_call falls back to CEK for Lambda/Component/Island values

Verified: (let ((x 10)) (let ((add-x (fn (y) (+ x y)))) (add-x 5)))
  Compiles to: CONST 10, LOC_SET #0, CLOSURE [UV_GET#0 LOC_GET#0 CPRIM+ RET]
  with upvalue descriptor: is_local=1 index=0
  VM executes → 15 ✓

Auto-compile: 6/117 functions compile (up from 3). Disabled until
compiler handles all features — fallback can't reconstruct closure
scope for variables like nav-state bound in caller's let*.

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

shared/sx/ocaml_bridge.py

@@ -265,9 +265,9 @@ class OcamlBridge:
             _logger.info("Loaded %d definitions from .sx files into OCaml kernel (%d skipped)",
                          count, skipped)
 
-            # VM auto-compile ready but disabled until compiler handles
-            # all SX features and CEK fallback works correctly.
-            # Enable with: await self._send('(vm-compile)')
+            # VM auto-compile: ready when compiler handles all SX features.
+            # Currently 6/117 compile; fallback fails on closure scope vars.
+            # await self._send('(vm-compile)')
         except Exception as e:
             _logger.error("Failed to load .sx files into OCaml kernel: %s", e)
             self._components_loaded = False  # retry next time