haskell: Phase 17 — import declarations anywhere among top-level decls

hk-collect-module-body previously ran a fixed import-loop at the start
and then a separate decl-loop; merged into a single hk-body-step
dispatcher that routes `import` to the imports list and everything else
to hk-parse-decl. Both call sites (initial step + post-semicolon loop)
use the dispatcher. The eval side reads imports as a list (not by AST
position) so mid-stream imports feed into hk-bind-decls! unchanged.

tests/parse-extras.sx 12 → 17: very-top, mid-stream, post-main,
two-imports-different-positions, unqualified-mid-file. Regression
sweep clean: eval 66/0, exceptions 14/0, typecheck 15/0, records 14/0,
ioref 13/0, map 26/0, set 17/0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/haskell/parser.sx

@@ -1733,10 +1733,18 @@
                   (= (hk-peek-type) "eof")
                   (hk-match? "vrbrace" nil)
                   (hk-match? "rbrace" nil))))
+            (define
+              hk-body-step
+              (fn
+                ()
+                (cond
+                  ((hk-match? "reserved" "import")
+                    (append! imports (hk-parse-import)))
+                  (:else (append! decls (hk-parse-decl))))))
             (when
               (not (hk-body-at-end?))
               (do
-                (append! decls (hk-parse-decl))
+                (hk-body-step)
                 (define
                   hk-body-loop
                   (fn
@@ -1747,7 +1755,7 @@
                         (hk-advance!)
                         (when
                           (not (hk-body-at-end?))
-                          (append! decls (hk-parse-decl)))
+                          (hk-body-step))
                         (hk-body-loop)))))
                 (hk-body-loop)))
             (list imports decls))))