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/eval.sx

@@ -370,11 +370,11 @@
         (cond
           ((dict-has? bindings pat) :scm-no-match)  ;; non-linear
           (:else (assoc bindings pat form))))
-      ;; pat is a list — match list-of-same-length
+      ;; pat is a list — delegate to scm-match-list, which itself
+      ;; handles ellipsis tail patterns where the lengths differ.
       ((list? pat)
         (cond
           ((not (list? form)) :scm-no-match)
-          ((not (= (length pat) (length form))) :scm-no-match)
           (:else (scm-match-list pat form literals bindings))))
       ;; literal value: must equal
       (:else
@@ -383,7 +383,30 @@
 (define scm-match-list
   (fn (pats forms literals bindings)
     (cond
-      ((or (nil? pats) (= (length pats) 0)) bindings)
+      ((or (nil? pats) (= (length pats) 0))
+        (cond
+          ((or (nil? forms) (= (length forms) 0)) bindings)
+          (:else :scm-no-match)))
+      ;; Ellipsis: (<pat> ... <rest>) — currently Phase 6b only
+      ;; supports a single ellipsis at the END of the pattern list.
+      ;; <pat> binds to the rest of the forms as a LIST.
+      ((and (>= (length pats) 2)
+            (string? (nth pats 1))
+            (= (nth pats 1) "..."))
+        (cond
+          ((not (= (length pats) 2))
+            ;; Tail-ellipsis only for now; nested or middle deferred.
+            :scm-no-match)
+          ((not (string? (first pats)))
+            ;; (<list-pat> ...) needs richer support — defer.
+            :scm-no-match)
+          (:else
+            ;; Bind first-pat to the remaining forms as a list.
+            (let ((name (first pats)))
+              (cond
+                ((dict-has? bindings name) :scm-no-match)
+                (:else (assoc bindings name forms)))))))
+      ((or (nil? forms) (= (length forms) 0)) :scm-no-match)
       (:else
         (let ((sub (scm-match-step (first pats) (first forms)
                                    literals bindings)))
@@ -402,6 +425,8 @@
 
 ;; Template instantiation: walk the template, substituting pattern
 ;; variables with their bindings; leave non-pattern-vars alone.
+;; Inside a list, a `<var> ...` pair splices the list-valued binding
+;; of <var> in place — matches the tail-ellipsis pattern shape.
 (define scm-instantiate
   (fn (tmpl bindings)
     (cond
@@ -410,9 +435,32 @@
       ((list? tmpl)
         (cond
           ((= (length tmpl) 0) tmpl)
-          (:else (map (fn (t) (scm-instantiate t bindings)) tmpl))))
+          (:else (scm-instantiate-list tmpl bindings))))
       (:else tmpl))))
 
+(define scm-instantiate-list
+  (fn (tmpl bindings)
+    (cond
+      ((or (nil? tmpl) (= (length tmpl) 0)) (list))
+      ;; <var> ... → splice the list-valued binding of <var>.
+      ((and (>= (length tmpl) 2)
+            (string? (nth tmpl 1))
+            (= (nth tmpl 1) "...")
+            (string? (first tmpl))
+            (dict-has? bindings (first tmpl)))
+        (scm-list-append-all
+          (get bindings (first tmpl))
+          (scm-instantiate-list (rest (rest tmpl)) bindings)))
+      (:else
+        (cons (scm-instantiate (first tmpl) bindings)
+              (scm-instantiate-list (rest tmpl) bindings))))))
+
+(define scm-list-append-all
+  (fn (xs ys)
+    (cond
+      ((or (nil? xs) (= (length xs) 0)) ys)
+      (:else (cons (first xs) (scm-list-append-all (rest xs) ys))))))
+
 ;; Try each rule against the form; return the instantiated template
 ;; or :scm-no-match if no rule matches.
 (define scm-expand