scheme: Phase 6a — define-syntax + syntax-rules (no ellipsis) + 12 tests

eval.sx adds macro infrastructure:
- {:scm-tag :macro :literals (LIT...) :rules ((PAT TMPL)...) :env E}
- scheme-macro? predicate
- scm-match / scm-match-list — pattern matching against literals,
  pattern variables, and structural list shapes
- scm-instantiate — template substitution with bindings
- scm-expand-rules — try each rule in order
- (syntax-rules (LITS) (PAT TMPL)...) → macro value
- (define-syntax NAME FORM) → bind macro in env
- scheme-eval: when head looks up to a macro, expand and re-eval

Pattern matching supports:
- _ → match anything, no bind
- literal symbols from the LITERALS list → must equal-match
- other symbols → pattern variables, bind to matched form
- list patterns → must be same length, each element matches

NO ellipsis (`...`) support yet — that's Phase 6b. NO hygiene
yet (introduced symbols can shadow caller bindings) — that's
Phase 6c, which will be the second consumer for
lib/guest/reflective/hygiene.sx.

12 tests cover: simple substitution, multi-rule selection,
nested macro use, swap-idiom (state mutation via set!), control-
flow wrappers, literal-keyword pattern matching, macros inside
lambdas.

249 total Scheme tests now (62 + 23 + 49 + 78 + 25 + 12).

lib/scheme/tests/macros.sx

@@ -0,0 +1,105 @@
+;; lib/scheme/tests/macros.sx — define-syntax + syntax-rules.
+
+(define scm-mac-pass 0)
+(define scm-mac-fail 0)
+(define scm-mac-fails (list))
+
+(define
+  scm-mac-test
+  (fn
+    (name actual expected)
+    (if
+      (= actual expected)
+      (set! scm-mac-pass (+ scm-mac-pass 1))
+      (begin
+        (set! scm-mac-fail (+ scm-mac-fail 1))
+        (append! scm-mac-fails {:name name :actual actual :expected expected})))))
+
+(define
+  scm-mac
+  (fn
+    (src)
+    (scheme-eval-program (scheme-parse-all src) (scheme-standard-env))))
+
+;; ── Basic syntax-rules ──────────────────────────────────────────
+
+(scm-mac-test
+  "my-if true"
+  (scm-mac
+    "(define-syntax my-if (syntax-rules () ((_ c t e) (cond (c t) (else e)))))\n     (my-if #t 'yes 'no)")
+  "yes")
+(scm-mac-test
+  "my-if false"
+  (scm-mac
+    "(define-syntax my-if (syntax-rules () ((_ c t e) (cond (c t) (else e)))))\n     (my-if #f 'yes 'no)")
+  "no")
+(scm-mac-test
+  "double"
+  (scm-mac
+    "(define-syntax double (syntax-rules () ((_ x) (+ x x))))\n     (double 21)")
+  42)
+(scm-mac-test
+  "nested macro use"
+  (scm-mac
+    "(define-syntax double (syntax-rules () ((_ x) (+ x x))))\n     (double (double 5))")
+  20)
+
+;; ── Macro with multiple rules ───────────────────────────────────
+
+(scm-mac-test
+  "multi-rule: matches first"
+  (scm-mac
+    "(define-syntax twin (syntax-rules () ((_ a) a) ((_ a b) (+ a b))))\n     (twin 7)")
+  7)
+(scm-mac-test
+  "multi-rule: matches second"
+  (scm-mac
+    "(define-syntax twin (syntax-rules () ((_ a) a) ((_ a b) (+ a b))))\n     (twin 3 4)")
+  7)
+
+;; ── Macros wrapping control flow ────────────────────────────────
+
+(scm-mac-test
+  "swap idiom"
+  (scm-mac
+    "(define-syntax swap! (syntax-rules () ((_ a b) (let ((tmp a)) (set! a b) (set! b tmp)))))\n     (define x 1) (define y 2)\n     (swap! x y)\n     (list x y)")
+  (list 2 1))
+
+;; ── Macros that expand to expressions, not values ──────────────
+
+(scm-mac-test
+  "my-unless: true → empty"
+  (scm-mac
+    "(define-syntax my-unless (syntax-rules () ((_ c body) (if c 'skipped body))))\n     (my-unless #t 99)")
+  "skipped")
+(scm-mac-test
+  "my-unless: false → body"
+  (scm-mac
+    "(define-syntax my-unless (syntax-rules () ((_ c body) (if c 'skipped body))))\n     (my-unless #f 99)")
+  99)
+
+;; ── Macro with literal keyword ─────────────────────────────────
+
+(scm-mac-test
+  "literal: => recognised"
+  (scm-mac
+    "(define-syntax tag-arrow (syntax-rules (=>) ((_ a => b) (list 'arrow a b))))\n     (tag-arrow 1 => 2)")
+  (list "arrow" 1 2))
+
+;; ── Macro keyword passed through unevaluated ────────────────────
+
+(scm-mac-test
+  "list expansion preserves arg order"
+  (scm-mac
+    "(define-syntax tuple (syntax-rules () ((_ a b c) (list a b c))))\n     (tuple 1 2 3)")
+  (list 1 2 3))
+
+;; ── Macros + lambdas ────────────────────────────────────────────
+
+(scm-mac-test
+  "macro inside lambda"
+  (scm-mac
+    "(define-syntax sq (syntax-rules () ((_ x) (* x x))))\n     (define (f n) (+ (sq n) 1))\n     (f 5)")
+  26)
+
+(define scm-mac-tests-run! (fn () {:total (+ scm-mac-pass scm-mac-fail) :passed scm-mac-pass :failed scm-mac-fail :fails scm-mac-fails}))