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

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simtrace-analyser-v82';
+const CACHE = 'simtrace-analyser-v83';
 
 const PRECACHE = [
     'index.html',