Fix url_decode + in form data; parser uses do (not begin)

- sx_server.ml: url_decode now decodes + as space (RFC 1866)
- parser.sx: changed begin block to do (no behavioral difference)
- handler: clean compile handler with source param

NOTE: hs-parse still returns (do) in ASER mode. Mutable closures
work (counter test passes), tokenizer works (JIT OK), but
hs-parse's 50+ define chain inside a single let fails in ASER.
Investigating as a separate evaluator issue.

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

lib/hyperscript/parser.sx

@@ -512,7 +512,7 @@
       (define
         parse-go-cmd
         (fn () (match-kw "to") (list (quote go) (parse-expr))))
-      (begin
+      (do
         (define
           parse-arith
           (fn