Walks every sidebar page against an older, released chain binary
(hermetic fixture hub) and fails when any page claims 'unreachable'
although the hub answers, or renders raw error text. Run via the
platform repo's scripts/skew-smoke.sh as a Studio release gate.
Verified against the real pair hub 0.21.0 vs workspace 0.22.0: all
invariants hold. Two timing notes are handled explicitly: real-async
settles between navigations and an app teardown before the fixture
teardown keep grpc-dart's data-after-cancel race (unguarded
_responses.add, call.dart:395) from failing the walk spuriously;
the final fake-time pump drains the channel's 5-minute idle timer.
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>
