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.

0 files changed, 0 insertions, 0 deletions