The plugin now works on macOS. Results are unchanged from 1.2.2 for
the same board and settings - nothing in the numerics was touched;
this release is platform fixes and per-OS documentation.

macOS (field-tested on KiCad 10):

- Fixed a crash on launch. KiCad's macOS builds bundle Python 3.9,
  and one module's type annotations were evaluated at import there
  ("unsupported operand type(s) for |: 'type' and 'NoneType'"). The
  plugin now runs on 3.9, and a test walks every shipped module so
  the incompatibility cannot silently return.
- Fixed every figure - the error figure included - refusing to render
  with "Cannot load backend 'TkAgg' ... as 'qt' is currently
  running". macOS' bundled Python ships tkinter, so matplotlib
  preferred Tk while the selection dialog had already made the
  process a Qt one. Qt (PySide6, a hard dependency) is now always
  the first choice on every platform.
- A board in a read-only location - such as the demo projects opened
  straight from the mounted installer image - no longer kills the run
  when the results directory cannot be created next to the board.
  Results fall back to a temp directory and the path is printed to
  the Messages panel.
- The test suite additionally runs against the stack a Mac plugin
  environment actually resolves (Python 3.9, numpy 2.0, scipy 1.13,
  matplotlib 3.9, PySide6 6.10) - 140 tests on both stacks.

Linux:

- On ARM64 (aarch64) the plugin environment could never build: pyamg
  publishes no wheels for that platform, KiCad installs wheels only,
  and one unresolvable requirement fails the whole environment.
  pyamg is now skipped there and the solver falls back to Jacobi-CG -
  same results, noticeably slower on large grids. Linux as a whole
  remains untested; reports welcome.

Documentation:

- Setup now gives dedicated instructions per operating system: which
  interpreter path to check, how to deploy, and where the plugin's
  Python environment lives on Windows, macOS and Linux (for the
  delete-and-restart recovery). A platform-notes section records what
  is actually tested on each OS and what to expect there.