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>

spec/tests/test-modules.sx

@@ -0,0 +1,91 @@
+;; R7RS module system tests — define-library / import
+
+(defsuite
+  "define-library-basic"
+  (deftest
+    "define and import a library"
+    (define-library
+      (test math)
+      (export add square)
+      (begin
+        (define add (fn (a b) (+ a b)))
+        (define square (fn (x) (* x x)))))
+    (import (test math))
+    (assert= (add 3 4) 7)
+    (assert= (square 5) 25))
+  (deftest
+    "library isolation — internal not exported"
+    (define-library
+      (test internal)
+      (export public-fn)
+      (begin
+        (define helper (fn (x) (* x 2)))
+        (define public-fn (fn (x) (helper (+ x 1))))))
+    (import (test internal))
+    (assert= (public-fn 5) 12))
+  (deftest
+    "multiple libraries"
+    (define-library
+      (test greet)
+      (export greet)
+      (begin (define greet (fn (name) (str "Hello, " name "!")))))
+    (define-library
+      (test format)
+      (export shout)
+      (begin (define shout (fn (s) (upper s)))))
+    (import (test greet))
+    (import (test format))
+    (assert= (greet "world") "Hello, world!")
+    (assert= (shout "hello") "HELLO")))
+
+(defsuite
+  "import-qualifiers"
+  (deftest
+    "import with only"
+    (define-library
+      (test utils)
+      (export inc dec double)
+      (begin
+        (define inc (fn (x) (+ x 1)))
+        (define dec (fn (x) (- x 1)))
+        (define double (fn (x) (* x 2)))))
+    (import (only (test utils) inc double))
+    (assert= (inc 5) 6)
+    (assert= (double 5) 10))
+  (deftest
+    "import with prefix"
+    (define-library
+      (test prefixed)
+      (export value)
+      (begin (define value 42)))
+    (import (prefix (test prefixed) tp:))
+    (assert= tp:value 42)))
+
+(defsuite
+  "library-registry"
+  (deftest
+    "library-loaded? returns true after define"
+    (define-library (test check) (export x) (begin (define x 1)))
+    (assert (library-loaded? (quote (test check)))))
+  (deftest
+    "library-loaded? returns false for unknown"
+    (assert (not (library-loaded? (quote (nonexistent lib)))))))
+
+(defsuite
+  "import-suspension"
+  (deftest
+    "import of unknown library suspends"
+    (let
+      ((state (cek-step-loop (make-cek-state (quote (import (remote data))) (make-env) (list)))))
+      (assert (cek-suspended? state))
+      (assert= (get (cek-io-request state) "op") "import")))
+  (deftest
+    "import suspension resumes after library registered"
+    (let
+      ((state (cek-step-loop (make-cek-state (quote (begin (import (lazy lib)) (get-value))) (make-env) (list)))))
+      (assert (cek-suspended? state))
+      (register-library (quote (lazy lib)) {:get-value (fn () 42)})
+      (let
+        ((final (cek-resume state nil)))
+        (assert (cek-terminal? final))
+        (assert= (cek-value final) 42)))))