HS: fetch URL parser fix + IO mock responses

Parser: handle /path URLs in fetch command by reading /+ident tokens.
Test runner: mock fetch routes (/test→yay, /test-json→{"foo":1}),
  io-parse-text, io-parse-json, io-parse-html handlers in _driveAsync.

Fetch tests still fail (0/23) because the do-block halts after
hs-fetch's perform suspension — the CEK machine doesn't continue
to the next command (put it into me) after IO resume. This needs
the IO suspension model to properly chain do-block continuations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/hyperscript/parser.sx

@@ -1266,7 +1266,7 @@
                   ((url (if (and (= (tp-type) "keyword") (= (tp-val) "from")) (do (adv!) (parse-arith (parse-poss (parse-atom)))) nil)))
                   (list (quote fetch-gql) gql-source url))))
             (let
-              ((url-atom (parse-atom)))
+              ((url-atom (if (and (= (tp-type) "op") (= (tp-val) "/")) (do (adv!) (let ((path-parts (list "/"))) (define read-path (fn () (when (and (not (at-end?)) (or (= (tp-type) "ident") (= (tp-type) "op") (= (tp-type) "dot") (= (tp-type) "number"))) (append! path-parts (tp-val)) (adv!) (read-path)))) (read-path) (join "" path-parts))) (parse-atom))))
               (let
                 ((url (if (nil? url-atom) url-atom (parse-arith (parse-poss url-atom)))))
                 (let