feat(config-upgrade-snapshot): add bug-857 regression test, correct design.md (task 8.5)

Prove Requirement 6.2 with a real, code-backed mechanism instead of the
originally hypothesized lost_and_found re-homing, which turned out to be
aspirational and never implemented (confirmed by exhaustive grep at both
the pre-fix baseline commit and HEAD — bug-857 itself was flagged in
buglog.json as never empirically reproduced).

test_bug857_regression.c demonstrates both halves of the requirement
entirely within controller_unit_test, no live soak or old-commit build
needed: schedule_checkpoint's pre-existing WAL-threshold trigger is
decoupled from the config-marker boundary and produces genuinely
divergent per-member checkpoint content, while the marker-triggered
checkpoint this spec added converges members regardless of differing
per-member processing pace.

design.md's Requirements Traceability table and Testing Strategy section
are corrected to match; the redirection reasoning is recorded in
tasks.md's Implementation Notes for the benefit of tasks 8.6-8.9, which
still need live multi-process HA-soak infrastructure for Requirement 6.1
that this task deliberately did not build.

src/controller/meson.build

@@ -176,6 +176,7 @@ controller_unit_test_src = [
   'testsuite/test_job_array_replay_fence.c',
   'testsuite/test_upgrade_snapshot_restore.c',
   'testsuite/test_upgrade_snapshot_apply.c',
+  'testsuite/test_bug857_regression.c',
   'ctrlstatus.c',
   'commit.c',
   'job.c',