Step 5p6 lazy loading + Step 6b VM transpilation prep

Lazy module loading (Step 5 piece 6 completion):
- Add define-library wrappers + import declarations to 13 source .sx files
- compile-modules.js generates module-manifest.json with dependency graph
- compile-modules.js strips define-library/import before bytecode compilation
  (VM doesn't handle these as special forms)
- sx-platform.js replaces hardcoded 24-file loadWebStack() with manifest-driven
  recursive loader — only downloads modules the page needs
- Result: 12 modules loaded (was 24), zero errors, zero warnings
- Fallback to full load if manifest missing

VM transpilation prep (Step 6b):
- Refactor lib/vm.sx: 20 accessor functions replace raw dict access
- Factor out collect-n-from-stack, collect-n-pairs, pad-n-nils helpers
- bootstrap_vm.py: transpiles 9 VM logic functions to OCaml
- sx_vm_ref.ml: proof that vm.sx transpiles (preamble has stubs)

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

web/harness-reactive.sx

@@ -1,3 +1,7 @@
+(define-library (sx harness-reactive)
+  (export assert-signal-value assert-signal-has-subscribers assert-signal-no-subscribers assert-signal-subscriber-count simulate-signal-set! simulate-signal-swap! assert-computed-dep-count assert-computed-depends-on count-effect-runs make-test-signal assert-batch-coalesces)
+  (begin
+
 (define
   assert-signal-value
   :effects ()
@@ -108,3 +112,9 @@
           expected-notify-count
           " notifications, got "
           notify-count)))))
+
+
+))
+
+;; Re-export to global env
+(import (sx harness-reactive))