Step 5 piece 6: migrate 23 .sx files to define-library/import

Wraps all core .sx files in R7RS define-library with explicit export
lists, plus (import ...) at end for backward-compatible global re-export.

Libraries registered:
  (sx bytecode)      — 83 opcode constants
  (sx render)        — 15 tag registries + render helpers
  (sx signals)       — 23 reactive signal primitives
  (sx r7rs)          — 21 R7RS aliases
  (sx compiler)      — 42 compiler functions
  (sx vm)            — 32 VM functions
  (sx freeze)        — 9 freeze/thaw functions
  (sx content)       — 6 content store functions
  (sx callcc)        — 1 call/cc wrapper
  (sx highlight)     — 13 syntax highlighting functions
  (sx stdlib)        — 47 stdlib functions
  (sx swap)          — 13 swap algebra functions
  (sx render-trace)  — 8 render trace functions
  (sx harness)       — 21 test harness functions
  (sx canonical)     — 12 canonical serialization functions
  (web adapter-html) — 13 HTML renderer functions
  (web adapter-sx)   — 13 SX wire format functions
  (web engine)       — 33 hypermedia engine functions
  (web request-handler) — 4 request handling functions
  (web page-helpers) — 12 page helper functions
  (web router)       — 36 routing functions
  (web deps)         — 19 dependency analysis functions
  (web orchestration) — 59 page orchestration functions

Key changes:
- define-library now inherits parent env (env-extend env instead of
  env-extend make-env) so library bodies can access platform primitives
- sx_server.ml: added resolve_library_path + load_library_file for
  import resolution (maps library specs to file paths)
- cek_run_with_io: handles "import" locally instead of sending to
  Python bridge

2608/2608 tests passing.

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

lib/vm.sx

@@ -1,3 +1,41 @@
+
+
+(define-library (sx vm)
+  (export
+    make-upvalue-cell
+    uv-get
+    uv-set!
+    make-vm-code
+    make-vm-closure
+    make-vm-frame
+    make-vm
+    vm-push
+    vm-pop
+    vm-peek
+    frame-read-u8
+    frame-read-u16
+    frame-read-i16
+    vm-push-frame
+    code-from-value
+    vm-closure?
+    vm-call
+    frame-local-get
+    frame-local-set
+    frame-upvalue-get
+    frame-upvalue-set
+    vm-global-get
+    vm-resolve-ho-form
+    vm-call-external
+    vm-global-set
+    env-walk
+    env-walk-set!
+    vm-create-closure
+    vm-run
+    vm-step
+    vm-call-closure
+    vm-execute-module)
+  (begin
+
 (define make-upvalue-cell (fn (value) {:uv-value value}))
 
 (define uv-get (fn (cell) (get cell "uv-value")))
@@ -556,3 +594,9 @@
         (dict-set! vm "frames" (list frame))
         (vm-run vm)
         (vm-pop vm)))))
+
+
+)) ;; end define-library
+
+;; Re-export to global namespace for backward compatibility
+(import (sx vm))