flow: retry combinator — re-run node on raised exceptions + 6 tests

(retry n node) re-runs up to n attempts on a raised exception; the last attempt's
exception propagates. Explicit (fail ...) values are NOT retried — they pass through.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/flow/spec.sx

@@ -24,6 +24,10 @@
 ;;   (fail-reason x)    — the reason carried by a failure value
 ;;   (try-catch node handler) — run node; if it raises, call (handler error)
 ;;     with the reified error and return the handler's value
+;;   (retry n node)     — run node, re-running up to n attempts total on a raised
+;;     exception; the last attempt's exception propagates. Only RAISED exceptions
+;;     are retried — explicit (fail ...) values pass through unchanged. (Once a
+;;     node has suspended in Phase 3, retry does not re-run it; resume continues.)
 
 (define
   flow-combinators-src
@@ -31,7 +35,7 @@
 
 (define
   flow-control-src
-  "(define (branch pred then else)\n     (lambda (input) (if (pred input) (then input) (else input))))\n   (define (fail reason) (list (quote flow-fail) reason))\n   (define (failed? x) (and (pair? x) (eq? (car x) (quote flow-fail))))\n   (define (fail-reason x) (car (cdr x)))\n   (define (try-catch node handler)\n     (lambda (input) (guard (e (#t (handler e))) (node input))))")
+  "(define (branch pred then else)\n     (lambda (input) (if (pred input) (then input) (else input))))\n   (define (fail reason) (list (quote flow-fail) reason))\n   (define (failed? x) (and (pair? x) (eq? (car x) (quote flow-fail))))\n   (define (fail-reason x) (car (cdr x)))\n   (define (try-catch node handler)\n     (lambda (input) (guard (e (#t (handler e))) (node input))))\n   (define (flow-retry-step n node input)\n     (guard (e (#t (if (<= n 1) (raise e) (flow-retry-step (- n 1) node input))))\n       (node input)))\n   (define (retry n node) (lambda (input) (flow-retry-step n node input)))")
 
 (define
   flow-load-combinators!