HS: do→let/it chaining, single-IO fetch, fetch URL parser, IO mock

Compiler: do-blocks now compile to (let ((it cmd1)) (let ((it cmd2)) ...))
instead of (do cmd1 cmd2 ...). This chains the `it` variable through
command sequences, enabling `fetch X then put it into me` pattern.
Each command's result is bound to `it` for the next command.

Runtime: hs-fetch simplified to single perform (io-fetch url format)
instead of two-stage io-fetch + io-parse-text/json.

Parser: fetch URL /path handled by reading /+ident tokens.
Default fetch format changed to "text" (was "json").

Test runner: mock fetch routes with format-specific responses.
io-fetch handler returns content directly based on format param.

Fetch tests still need IO suspension to chain through let continuations.

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

lib/hyperscript/compiler.sx

@@ -999,7 +999,23 @@
                     (hs-to-sx (nth ast 1))
                     (hs-to-sx (nth ast 2)))))
               ((= head (quote do))
-                (cons (quote do) (map hs-to-sx (rest ast))))
+                (let
+                  ((compiled (map hs-to-sx (rest ast))))
+                  (if
+                    (= (len compiled) 1)
+                    (first compiled)
+                    (let
+                      ((last-cmd (nth compiled (- (len compiled) 1)))
+                        (init-cmds (reverse (rest (reverse compiled)))))
+                      (reduce
+                        (fn
+                          (body cmd)
+                          (list
+                            (quote let)
+                            (list (list (quote it) cmd))
+                            body))
+                        last-cmd
+                        (reverse init-cmds))))))
               ((= head (quote wait)) (list (quote hs-wait) (nth ast 1)))
               ((= head (quote wait-for)) (emit-wait-for ast))
               ((= head (quote log))

lib/hyperscript/parser.sx

@@ -1270,7 +1270,7 @@
               (let
                 ((url (if (nil? url-atom) url-atom (parse-arith (parse-poss url-atom)))))
                 (let
-                  ((fmt (if (match-kw "as") (let ((f (tp-val))) (adv!) f) "json")))
+                  ((fmt (if (match-kw "as") (let ((f (tp-val))) (adv!) f) "text")))
                   (list (quote fetch) url fmt)))))))
       (define
         parse-call-args

lib/hyperscript/runtime.sx

@@ -285,13 +285,7 @@
   hs-fetch
   (fn
     (url format)
-    (let
-      ((response (perform (list (quote io-fetch) url))))
-      (cond
-        ((= format "json") (perform (list (quote io-parse-json) response)))
-        ((= format "text") (perform (list (quote io-parse-text) response)))
-        ((= format "html") (perform (list (quote io-parse-html) response)))
-        (true response)))))
+    (perform (list "io-fetch" url (if format format "text")))))
 
 (define
   hs-coerce