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/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