scheme: Phase 8 — define-library + import (minimal) + 7 tests

eval.sx adds module support:

  (define-library NAME EXPR...)
    Where EXPR is one of:
      (export NAME ...)
      (import LIB-NAME ...)
      (begin BODY ...)

  (import LIB-NAME ...)
    Looks up each library by key, copies its exported names
    into the current env.

Library values: {:scm-tag :library :name :exports :env}
Stored in scheme-library-registry keyed by joined library-name
(`(my math)` → `"my/math"`).

Library body runs in a FRESH standard env (each library is its
own namespace). Only :exports are visible after import; private
internal definitions stay in the library's env. Internal calls
between library functions use the library's env, so public-facing
exports can rely on private helpers.

Multiple imports work — each library is independent.

NOT yet supported: cond-expand, include, include-library-
declarations, renaming (`(only ...)`, `(except ...)`, `(prefix ...)`,
`(rename ...)`). Standard R7RS modules use these but the core
two-operation flow (define-library / import) covers most everyday
module use.

7 tests: single export, multi-export, private-not-visible,
internal-calls-private, two-libs-both-imported, unknown-lib-error,
single-symbol library name.

296 total Scheme tests (62+23+49+78+25+20+13+10+9+7).

Phases done: 1, 2, 3, 3.5, 4, 5abc, 6ab, 7, 8, 9, 10.
Deferred: 6c (hygiene/scope-set — research-grade), 11 (conformance).

lib/scheme/tests/modules.sx

@@ -0,0 +1,73 @@
+;; lib/scheme/tests/modules.sx — define-library + import.
+
+(define scm-mod-pass 0)
+(define scm-mod-fail 0)
+(define scm-mod-fails (list))
+
+(define
+  scm-mod-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! scm-mod-pass (+ scm-mod-pass 1))
+      (begin
+        (set! scm-mod-fail (+ scm-mod-fail 1))
+        (append! scm-mod-fails {:name name :actual actual :expected expected})))))
+
+(define
+  scm-mod
+  (fn
+    (src)
+    (scheme-eval-program (scheme-parse-all src) (scheme-standard-env))))
+
+;; ── Basic define-library + import ───────────────────────────────
+
+(scm-mod-test
+  "simple lib: sq exported"
+  (scm-mod
+    "(define-library (my math)\n       (export sq)\n       (begin (define (sq x) (* x x))))\n     (import (my math))\n     (sq 5)")
+  25)
+(scm-mod-test
+  "lib: multiple exports"
+  (scm-mod
+    "(define-library (my math)\n       (export sq cube)\n       (begin\n         (define (sq x) (* x x))\n         (define (cube x) (* x x x))))\n     (import (my math))\n     (list (sq 5) (cube 3))")
+  (list 25 27))
+(scm-mod-test
+  "lib: single-symbol name"
+  (scm-mod
+    "(define-library (utils)\n       (export greet)\n       (begin (define (greet name) (string-append \"hi \" name))))\n     (import (utils))\n     (string=? (greet \"world\") \"hi world\")")
+  true)
+
+;; ── Unexported names are not visible ───────────────────────────
+
+(scm-mod-test
+  "lib: private name not exported"
+  (scm-mod
+    "(define-library (my math)\n       (export sq)\n       (begin\n         (define (sq x) (* x x))\n         (define (private-helper x) (+ x 1))))\n     (import (my math))\n     (guard (e (else 'unbound)) private-helper)")
+  "unbound")
+
+;; ── Library calls its own internals ─────────────────────────────
+
+(scm-mod-test
+  "lib: internal calls private fn"
+  (scm-mod
+    "(define-library (my math)\n       (export public-add1)\n       (begin\n         (define (private-inc x) (+ x 1))\n         (define (public-add1 x) (private-inc x))))\n     (import (my math))\n     (public-add1 41)")
+  42)
+
+;; ── Two libs, both imported ────────────────────────────────────
+
+(scm-mod-test
+  "two libs: both imported"
+  (scm-mod
+    "(define-library (a) (export af) (begin (define (af) 1)))\n     (define-library (b) (export bf) (begin (define (bf) 2)))\n     (import (a) (b))\n     (+ (af) (bf))")
+  3)
+
+;; ── Unknown library import errors ──────────────────────────────
+
+(scm-mod-test
+  "import: unknown lib errors"
+  (scm-mod "(guard (e (else 'unknown-lib)) (import (no such lib)))")
+  "unknown-lib")
+
+(define scm-mod-tests-run! (fn () {:total (+ scm-mod-pass scm-mod-fail) :passed scm-mod-pass :failed scm-mod-fail :fails scm-mod-fails}))