PingerPlot 1.3.1

PingerPlot 1.3.1

Adds downloadable Windows executables, and fixes a config-file bug found while
building them.

Added

• Windows executables attached to the release, built by CI from this commit:
  PingerPlot.exe (GUI) and pingerplot-headless.exe (console), plus
  SHA256SUMS.txt. Neither needs Python installed. Both are smoke-tested before
  they are attached: the GUI binary's import graph is verified through a new
  --version flag, and the headless binary runs a real ICMP probe. They are
  UNSIGNED -- see the README for what SmartScreen and antivirus heuristics will
  make of that, and why this binary is a worse-than-average case for the latter.
• main.py --version.

Fixed

• A byte-order mark made a config unreadable. Writing monitor.json with
  PowerShell's Set-Content -Encoding utf8 -- the shell that ships with Windows
  -- produced a file the app refused outright with "Unexpected UTF-8 BOM", exit
  1. Files a person may have authored are now read as utf-8-sig. The settings
     case was the quietest of these: load() swallows the decode error and returns
     {}, so a BOM there silently reset the theme, engine options, alert thresholds
     and target list to defaults with no message at all.

Changed

• CI opens net.ipv4.ping_group_range on the Linux runners. Without it the POSIX
  backend's live tests skipped on every ubuntu leg, so the backend shipped in
  1.3.0 having never executed a single socket operation anywhere. They now run
  against real sockets on real Linux. A real multi-hop path remains untested.