fix: preserve agent panel scroll when switching machines (#4101)

refs #3937

Root cause:
Activating another endpoint applies a snapshot with a different
endpoint-qualified graphics scope. This triggers reset_endpoint_projection,
which clears agent_scroll together with endpoint-local state even though
the agent list aggregates all machines. The selected agent receives focus,
but the list jumps back to the top.

Fix:
Save the client-owned agent scroll offset and restore it only when
activation changes endpoints. Keep same-endpoint reboot resets,
workspace/tab scroll resets, input cleanup and existing list bounds
clamping unchanged. This adds constant-time work on endpoint activation,
with no protocol, persisted-state or pane-scaled rendering changes.

Validation:
The new regression fails on the original implementation with scroll 0
instead of 6. Tests cover repeated bidirectional agent clicks, stable
visible rows, shrinking lists and same-machine reboots. All 242 client
shell tests pass.

Real SSH A/B testing against the same two disposable 0.9.0 servers
reproduces the reset with both the stable and current-master clients.
The patched client preserves the visible rows through six cross-machine
switches and two same-machine control clicks.

Native just ci passes: 3535 Rust tests, six default skips, plus lint,
maintenance, architecture and integration-asset checks. All seven docs
contract tests pass. Windows cross-lint was not run because its SDK is
not installed; native macOS and Windows UI behavior was not tested.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/client/shell/endpoints.rs b/src/client/shell/endpoints.rs
index 420f7131..d3823d54 100644
--- a/src/client/shell/endpoints.rs
+++ b/src/client/shell/endpoints.rs
@@ -208,12 +208,18 @@ impl ClientShellState {
         let Some(snapshot) = endpoint.snapshot.clone() else {
             return false;
         };
-        if endpoint_id != &self.active_endpoint_id {
+        let switching_endpoint = endpoint_id != &self.active_endpoint_id;
+        let agent_scroll = self.agent_scroll;
+        if switching_endpoint {
             self.active_endpoint_id = endpoint_id.clone();
             self.pane_surface = None;
             self.pending_pane_surface = None;
         }
         self.apply_active_snapshot(snapshot);
+        if switching_endpoint {
+            // The aggregate agent list belongs to the client, not one endpoint.
+            self.agent_scroll = agent_scroll;
+        }
         true
     }