scheme: Phase 6b — syntax-rules ellipsis (tail-rest) + 8 tests

scm-match-list now detects `<pat> ...` at the END of a pattern list
and binds <pat> (must be a symbol — single-variable rest) to the
remaining forms as a list. Nested-list patterns under ellipsis and
middle-of-list ellipses are NOT supported yet (rare in practice;
deferred).

scm-instantiate-list mirrors: when it encounters `<var> ... `
inside a list template, it splices the list-valued binding of <var>
in place. Internal list-append-all helper for the splice.

Removes the `(length pat) = (length form)` strict-equality check
in scm-match-step's list case — that gate blocked ellipsis. The
length-1-or-more relaxed check now lives in scm-match-list itself.

8 ellipsis tests cover:
- Empty rest (my-list)
- Non-empty rest (my-list 1 2 3 4)
- my-when with multi-body
- Variadic sum-em via fold-left
- Recursive my-and pattern (short-circuit AND defined as macro)

257 total Scheme tests (62 + 23 + 49 + 78 + 25 + 20).

Phase 6c (proper hygiene) is the next step and will be the
**second consumer for lib/guest/reflective/hygiene.sx** — the
deferred research-grade kit from the kernel-on-sx loop.

lib/scheme/tests/macros.sx

@@ -102,4 +102,54 @@
     "(define-syntax sq (syntax-rules () ((_ x) (* x x))))\n     (define (f n) (+ (sq n) 1))\n     (f 5)")
   26)
 
+;; ── Ellipsis patterns (Phase 6b — tail-rest single-variable) ────
+(scm-mac-test "ellipsis: empty rest"
+  (scm-mac
+    "(define-syntax my-list (syntax-rules () ((_ xs ...) (list xs ...))))
+     (my-list)")
+  (list))
+(scm-mac-test "ellipsis: list of values"
+  (scm-mac
+    "(define-syntax my-list (syntax-rules () ((_ xs ...) (list xs ...))))
+     (my-list 1 2 3 4)")
+  (list 1 2 3 4))
+(scm-mac-test "ellipsis: my-when truthy"
+  (scm-mac
+    "(define-syntax my-when (syntax-rules () ((_ c body ...) (if c (begin body ...)))))
+     (my-when #t 1 2 3)")
+  3)
+(scm-mac-test "ellipsis: my-when falsy returns nil"
+  (scm-mac
+    "(define-syntax my-when (syntax-rules () ((_ c body ...) (if c (begin body ...)))))
+     (my-when #f 1 2 3)")
+  nil)
+(scm-mac-test "ellipsis: begin-rebuild"
+  (scm-mac
+    "(define-syntax my-begin (syntax-rules () ((_ body ...) (let () body ...))))
+     (my-begin (define x 5) (define y 10) (+ x y))")
+  15)
+(scm-mac-test "ellipsis: variadic sum-em via fold"
+  (scm-mac
+    "(define-syntax sum-em (syntax-rules () ((_ xs ...) (fold-left + 0 (list xs ...)))))
+     (sum-em 1 2 3 4 5)")
+  15)
+(scm-mac-test "ellipsis: recursive my-and"
+  (scm-mac
+    "(define-syntax my-and
+       (syntax-rules ()
+         ((_) #t)
+         ((_ x) x)
+         ((_ x xs ...) (if x (my-and xs ...) #f))))
+     (my-and 1 2 3)")
+  3)
+(scm-mac-test "ellipsis: my-and short-circuits"
+  (scm-mac
+    "(define-syntax my-and
+       (syntax-rules ()
+         ((_) #t)
+         ((_ x) x)
+         ((_ x xs ...) (if x (my-and xs ...) #f))))
+     (my-and 1 #f 3)")
+  false)
+
 (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}))