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/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