fix(review): address PR #94 review #37/#38 reviewer comments

Parser (src/database_flatfile_parser.c):
- ff_jid_to_dir: drop GString+step1, in-place mutate str_replace result
- ff_ensure_dir: g_file_test(G_FILE_TEST_IS_SYMLINK) instead of g_lstat
- ff_unescape_*: early-continue for non-escape branch (less nesting)
- ff_readline: strdup("") instead of malloc(1)+'\0' for skip-line return
- ff_write_line: auto_char safe_msg + single fprintf (drop GString full_line)
- ff_parse_line: early return on missing closing bracket;
  extract metadata-parts loop into _ff_parse_meta_parts helper

Verify (src/database_flatfile_verify.c):
- Split monolithic ff_verify_integrity into _collect_contact_dirs,
  _check_permissions, _first_pass, _lmc_pass, _verify_contact_dir
- _issue_new constructor; g_new0 throughout
- Separate seen_stanza_ids and seen_archive_ids tables (no cross-kind
  duplicate confusion)
- ff_skip_bom in second pass (drop manual fgetc x3)
- log_debug when skipping a contact dir without history.log
- Doc comment on ff_verify_integrity in header

SQLite (src/database_sqlite.c):
- Doc block on ChatLogs schema (replaces_db_id <-> replaced_by_db_id
  invariant maintained by AFTER INSERT trigger)
- log_error when _get_db_filename returns NULL during init

Flatfile dispatch (src/database_flatfile.c):
- Replace if/else with is_outgoing ternary for log-path contact pick

Tests:
- 6 new invalid-date parser tests: empty / partial ISO / garbage /
  impossible calendar / negative year / far future timestamp
- New functional test message_db_history_multi_resource verifying
  same-JID-different-resource history rehydration preserves resources
- test_database_stress.c LMC chain loop: auto_char buf, drop redundant
  comment/empty pre-filter (ff_parse_line handles those)

tests/unittests/test_database_export.c

@@ -500,6 +500,66 @@ test_ff_parse_line_invalid_timestamp(void** state)
     assert_null(pl);
 }
 
+void
+test_ff_parse_line_empty_timestamp(void** state)
+{
+    /* The space before "[" makes the timestamp empty -> g_date_time_new_from_iso8601
+       must reject it. */
+    ff_parsed_line_t* pl = ff_parse_line(
+        " [chat|none] a@b.com: msg");
+    assert_null(pl);
+}
+
+void
+test_ff_parse_line_partial_iso_timestamp(void** state)
+{
+    /* Year-only or yyyy-mm without time portion is not ISO-8601 enough for
+       g_date_time_new_from_iso8601 — must fail rather than silently succeed. */
+    assert_null(ff_parse_line("2025 [chat|none] a@b.com: msg"));
+    assert_null(ff_parse_line("2025-06-15 [chat|none] a@b.com: msg"));
+}
+
+void
+test_ff_parse_line_garbage_timestamp(void** state)
+{
+    /* Random characters where timestamp should be — parse failure, no segfault. */
+    assert_null(ff_parse_line("@@@@@@ [chat|none] a@b.com: msg"));
+    assert_null(ff_parse_line("12345 [chat|none] a@b.com: msg"));
+}
+
+void
+test_ff_parse_line_impossible_calendar_date(void** state)
+{
+    /* Day 32 / Month 13 — well-formed ISO syntax but impossible date,
+       g_date_time_new_from_iso8601 returns NULL. */
+    assert_null(ff_parse_line(
+        "2025-13-01T12:00:00Z [chat|none] a@b.com: msg"));
+    assert_null(ff_parse_line(
+        "2025-06-32T12:00:00Z [chat|none] a@b.com: msg"));
+    assert_null(ff_parse_line(
+        "2025-06-15T25:00:00Z [chat|none] a@b.com: msg"));
+}
+
+void
+test_ff_parse_line_negative_year_timestamp(void** state)
+{
+    /* Negative year is not parseable by g_date_time_new_from_iso8601. */
+    assert_null(ff_parse_line(
+        "-0001-06-15T12:00:00Z [chat|none] a@b.com: msg"));
+}
+
+void
+test_ff_parse_line_far_future_timestamp(void** state)
+{
+    /* Year 9999 is valid ISO-8601 and should parse — sanity check that we
+       don't artificially reject valid far-future dates. */
+    ff_parsed_line_t* pl = ff_parse_line(
+        "9999-12-31T23:59:59Z [chat|none] a@b.com: hi");
+    assert_non_null(pl);
+    assert_non_null(pl->timestamp);
+    ff_parsed_line_free(pl);
+}
+
 /* ================================================================
  * Type / enc conversion round-trip
  * ================================================================ */