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/functionaltests/functionaltests.c

@@ -234,6 +234,7 @@ main(int argc, char* argv[])
         PROF_FUNC_TEST(message_db_history_service_chars),
         PROF_FUNC_TEST(message_db_history_verify),
         PROF_FUNC_TEST(message_db_history_lmc),
+        PROF_FUNC_TEST(message_db_history_multi_resource),
 
         /* Basic message send/receive */
         PROF_FUNC_TEST(message_send),