sx-gitea import: snapshot imports + custom commit message + remote replace

Import from an IMMUTABLE snapshot (git archive), not the live tree — a
replay diverges the moment a source file changes (the forge's own
non-fast-forward check caught exactly that). import-stage-msg! carries
the source SHA in the commit message; import-delete-remote! + push
replaces a partial import's history in two requests.

rose-ash mirrored to sx.sx-web.org/giles/rose-ash: 4468 files @
4a7c05a2, one commit, zero skipped, single push under the linear wire.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/gitea/import.sx

@@ -12,7 +12,8 @@
 ; without re-pushing), import-push! sends the delta between the remote's
 ; advertised head and the local one in a single request. Pushing per
 ; batch is quadratic (every push walks the full closure on both sides);
-; stage many, push once.
+; stage many, push once. Import from an IMMUTABLE snapshot (git archive)
+; — replaying against a live tree diverges the moment a file changes.
 ;
 ; Files whose pack line would exceed the pkt-line limit are skipped and
 ; reported per batch (wire limit; see lib/gitea/wire.sx).
@@ -54,12 +55,12 @@
   gitea/import-fits?
   (fn (data) (gitea/pkt-fits? (str "x" (serialize (git/blob data))))))
 
-; read + stage one manifest of paths and commit — NO push
-; => {:batch n :files k :skipped (paths) :cid commit-cid}
+; read + stage one manifest of paths and commit with the given message —
+; NO push. => {:batch n :files k :skipped (paths) :cid commit-cid}
 (define
-  gitea/import-stage!
+  gitea/import-stage-msg!
   (fn
-    (root manifest)
+    (root manifest message time)
     (let
       ((st gitea/import-state))
       (let
@@ -69,9 +70,21 @@
         (let
           ((skipped (reduce (fn (acc p) (let ((data (file-read (str root "/" p)))) (if (gitea/import-fits? data) (begin (git/add! repo p data) acc) (append acc (list p))))) (list) paths)))
           (let
-            ((cid (git/commit! repo {:message (str "import rose-ash: batch " n) :time n :author "giles"})))
+            ((cid (git/commit! repo {:message message :time time :author "giles"})))
             (begin (set! gitea/import-state (assoc st :batch n)) {:batch n :files (- (len paths) (len skipped)) :skipped skipped :cid cid})))))))
 
+(define
+  gitea/import-stage!
+  (fn
+    (root manifest)
+    (let
+      ((n (+ 1 (get gitea/import-state :batch))))
+      (gitea/import-stage-msg!
+        root
+        manifest
+        (str "import rose-ash: batch " n)
+        n))))
+
 ; one delta push of everything staged since the remote's advertised head
 (define
   gitea/import-push!
@@ -81,6 +94,15 @@
       ((st gitea/import-state))
       (gitea/push! (get st :remote) (get st :repo) "heads/main"))))
 
+; delete the remote branch (for replacing an import's history)
+(define
+  gitea/import-delete-remote!
+  (fn
+    ()
+    (let
+      ((st gitea/import-state))
+      (gitea/push-delete! (get st :remote) (get st :repo) "heads/main"))))
+
 ; stage + push in one step (fine for small imports)
 (define
   gitea/import-batch!