Release 1.3.0

macOS is now field-tested and working; the README platform notes say
so instead of untested. Linux remains audited-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/release-notes/v1.3.0.md

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+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.