test(functests): migration/MUC/timestamp tests, group rebalancing

Add 11 new export/import tests (24 → 35): 4 MUC database, 3 timestamp
verification, 3 migration stress, 1 LMC correction with negative assert.

Add delay-stamp timestamp checks to 5 base tests via /time chat set
iso8601 and /history on (previously passed by accident from PTY buffer).

Replace magic numbers: NEGATIVE_ASSERT_TIMEOUT, MIN_ELAPSED_INIT.
Add per-test slow/fast thresholds and TZ override (prof_test_tz).

Rebalance groups by whole logical blocks (spread 46s → 3s):
G1=52s G2=50s G3=53s G4=53s. All 129 tests pass.

tests/functionaltests/test_export_import.h

@@ -8,3 +8,17 @@ void import_double_dedup(void** state);
 void verify_after_export(void** state);
 void switch_backends_independent_messages(void** state);
 void export_empty_contact(void** state);
+void export_message_pool_dated(void** state);
+void export_lmc_pool_multiple(void** state);
+void export_large_body_survives(void** state);
+void bidirectional_merge_to_flatfile(void** state);
+void bidirectional_merge_to_sqlite(void** state);
+void migration_incremental_accumulation(void** state);
+void roundtrip_full_cycle(void** state);
+void export_timestamps_preserved(void** state);
+void import_timestamps_preserved(void** state);
+void export_timestamps_non_utc(void** state);
+void muc_export_sqlite_to_flatfile(void** state);
+void muc_import_flatfile_to_sqlite(void** state);
+void muc_export_timestamps_preserved(void** state);
+void muc_export_multiple_rooms(void** state);