Change details: no filler past a short file, no offsets for a single row (v2.7.3)

- the hex views padded every line to 16 bytes with '--', so a 2-byte file
  looked like it was missing data; the row now ends at the known file/record
  length (from the FCP — hexImageRows gained the size parameter), while '--'
  still marks bytes *inside* the file that were never observed (an unknown
  length keeps the old row padding, nothing better is available)
- the offset column is shown only when a view spans more than one row — a
  four-hex-digit 0000 above two bytes was noise
- the legend lists only the marks actually rendered: 'never observed' is gone
  from a short/fully-observed view, and 'newly observed' when nothing was
  first observed (the summary line still counts both)
- tests: pure hexImageRows cases (the 2-byte file, a 9-byte record with an
  unobserved tail, the unknown-size fallback) plus two renderFileDiff cases
  pinning the missing filler/offset and the trimmed legend; 527 Python +
  75 Node tests
- README/README_RUS describe the new hex-view behaviour;
  PWA_VERSION/pyproject/version.py 2.7.3, SW cache simtrace-analyser-v83

tests/test_pcap_import.py

@@ -224,7 +224,7 @@ class TestImportEndpoint(unittest.TestCase):
 class TestVersion(unittest.TestCase):
     def test_package_version(self):
         from simtrace2_pysniff import __version__
-        self.assertEqual(__version__, '2.7.2')
+        self.assertEqual(__version__, '2.7.3')
 
     def test_status_includes_version(self):
         import os