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/testsuite/controller_test.c

@@ -91,6 +91,7 @@ extern void test_job_array_fence(void);
 extern void test_job_array_replay_fence(void);
 extern void test_upgrade_snapshot_restore(void);
 extern void test_upgrade_snapshot_apply(void);
+extern void test_bug857_regression(void);
 
 void setUp(void) { return; }
 
@@ -251,6 +252,7 @@ int main(int argc, char* argv[])
     TEST(test_job_array_replay_fence);
     TEST(test_upgrade_snapshot_restore);
     TEST(test_upgrade_snapshot_apply);
+    TEST(test_bug857_regression);
 
     return UNITY_END();
 }