Step 5: CEK IO suspension + R7RS modules (define-library/import)

Third CEK phase "io-suspended": perform suspends evaluation, host
resolves IO, cek-resume feeds result back. VM OP_PERFORM (opcode 112)
enables JIT-compiled functions to suspend. VM→CEK→suspend chain
propagates suspension across the JIT/CEK boundary via pending_cek.

R7RS define-library creates isolated environments with export control.
import checks the library registry and suspends for unknown libraries,
enabling lazy on-demand loading. Import qualifiers: only, prefix.

Server-side cek_run_with_io handles suspension by dispatching IO
requests to the Python bridge and resuming. guard composes cleanly
with perform for structured error recovery across IO boundaries.

2598/2598 tests (30 new: 15 core suspension, 3 JIT, 1 cross-boundary,
9 modules, 2 error handling). Zero regressions.

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

hosts/javascript/platform.py

@@ -1714,7 +1714,8 @@ PLATFORM_CEK_JS = '''
 CEK_FIXUPS_JS = '''
   // Override recursive cekRun with iterative loop (avoids stack overflow)
   cekRun = function(state) {
-    while (!cekTerminal_p(state)) { state = cekStep(state); }
+    while (!cekTerminal_p(state) && !cekSuspended_p(state)) { state = cekStep(state); }
+    if (cekSuspended_p(state)) { throw new Error("IO suspension in non-IO context"); }
     return cekValue(state);
   };