test(host): a chain may write one record twice, and why that works

Two measures on one record is one record, so writing the same record twice in
a frame is both legitimate and likely. It works for a reason worth pinning
rather than rediscovering: **every check happens before any command is
applied**, so the second write is compared to the record's *pre-frame*
counter and not to the one its own sibling just bumped.

Were the checks interleaved with the applies — which is what the previous
commit's per-command record check would look like if it were moved into the
apply loop — the frame would refuse itself with `RecordMoved`. Proved by
injecting exactly that interleaving: this arm fails and nothing else does.

1291 passing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/improvise-command/src/ui/host.rs

@@ -881,6 +881,48 @@ mod tests {
                 .get_cell(&cell("South")),
             Some(&CellValue::Number(20.0)),
         );
+
+        // ── arm 3: the same record twice in one chain ───────────────────
+        // Legitimate and likely — two measures on one record is one record —
+        // and it works for a reason worth pinning rather than rediscovering:
+        // **every check happens before any command is applied**, so the
+        // second write is compared to the record's *pre-frame* counter, not
+        // to the one its own sibling just bumped. Were the checks
+        // interleaved with the applies, this frame would refuse itself with
+        // `RecordMoved`.
+        let mut twice = host();
+        let (asker, _) = twice.connect(10, 10);
+        twice.model(asker, write("North", 1.0));
+        let basis = frame_basis(&twice, &["North"]);
+        let answer = twice.handle(
+            asker,
+            ClientFrame::Commands(vec![
+                Command::Model {
+                    basis: basis.clone(),
+                    cmd: write("North", 10.0),
+                },
+                Command::Model {
+                    basis,
+                    cmd: write("North", 11.0),
+                },
+            ]),
+        );
+        assert!(
+            !answer
+                .iter()
+                .any(|a| matches!(a.msg, ServerFrame::Rejected(_))),
+            "a chain may write one record twice: {answer:?}"
+        );
+        assert_eq!(
+            twice
+                .app()
+                .model_state
+                .workbook
+                .model
+                .get_cell(&cell("North")),
+            Some(&CellValue::Number(11.0)),
+            "and the last write in the chain is the one that stands"
+        );
     }
 
     /// **The capability the contract is for**: a chain of data-dependent