test: cover collision and permission failures

e2e/README.md

@@ -45,7 +45,9 @@ source staging, and command replay after a target client disconnect. The final
 hostile cases change the source selection and make an absent target appear
 after confirmation; each must fail before commit with its exact precondition
 reason. qBittorrent is also checked for zero downloaded bytes at the replayed
-target.
+target. Finally, a wrong-size pre-existing target and an unwritable target root
+must fail without committing or losing the source; the permission fixture
+restores the target root's original mode even on an aborted run.
 
 The scheduler pause/resume endpoints exist only on the loopback test adapter;
 normal control and Telegram orchestration always use the automatic scheduler.

e2e/scripts/complex-matrix.sh

@@ -97,6 +97,15 @@ assert_qb() {
         --selected "$selected" >/dev/null
 }
 
+archive2_download_mode=
+restore_archive2_download_mode() {
+    if [[ -n "$archive2_download_mode" ]]; then
+        compose_node archive-2 exec -T qbittorrent \
+            chmod "$archive2_download_mode" /downloads >/dev/null 2>&1 || true
+    fi
+}
+trap restore_archive2_download_mode EXIT
+
 for node in cache-1 cache-2 archive-1 archive-2; do
     delete_torrent "$node" "$legacy_bundle_hash"
     delete_torrent "$node" "$bundle_hash"
@@ -377,6 +386,35 @@ control_scenario expect-failure \
 assert_qb cache-1 "$cancel_hash" '0'
 delete_torrent archive-2 "$cancel_hash"
 
+target_collision_job=$(control_scenario create-paused \
+    --operation archive \
+    --source-client cache-1 \
+    --target-client archive-2 \
+    --info-hash "$cancel_hash")
+compose_node archive-2 exec -T --user 1001:1001 qbittorrent /bin/sh -c \
+    'printf "wrong-size\n" > /downloads/fixture.bin'
+control_scenario expect-failure \
+    --job-id "$target_collision_job" \
+    --contains 'pre-existing destination size does not match'
+assert_qb cache-1 "$cancel_hash" '0'
+compose_node archive-2 exec -T qbittorrent rm -f /downloads/fixture.bin
+
+permission_job=$(control_scenario create-paused \
+    --operation archive \
+    --source-client cache-1 \
+    --target-client archive-2 \
+    --info-hash "$cancel_hash")
+archive2_download_mode=$(compose_node archive-2 exec -T qbittorrent \
+    stat -c '%a' /downloads)
+compose_node archive-2 exec -T qbittorrent chmod 0555 /downloads
+control_scenario expect-failure \
+    --job-id "$permission_job" \
+    --contains 'Permission denied'
+restore_archive2_download_mode
+archive2_download_mode=
+assert_qb cache-1 "$cancel_hash" '0'
+
 printf '%s\n' \
     'complex matrix passed: selective merges, union eviction, hostile retention,' \
-    'queued/partial cancellation, restart/replay, and hostile state changes'
+    'queued/partial cancellation, restart/replay, hostile state changes,' \
+    'target collisions, and permission denial'