Per-OS setup instructions; skip pyamg on Linux aarch64

The README assumed Windows throughout. Setup now gives the interpreter
path, deploy command and venv location for Windows, macOS and Linux
(venv paths verified against KiCad 10 sources: GetUserCachePath +
python-environments), plus a Platform notes section: macOS bundles
Python 3.9.13 so pip resolves an older wheel stack (KiCad installs with
--only-binary :all:), and windows there may open behind KiCad; Linux
needs python3-venv on Debian/Ubuntu.

pyamg has never published Linux aarch64 wheels, and with KiCad's
wheels-only pip one unresolvable requirement kills the whole venv
build - so an environment marker skips pyamg there and the solver
falls back to Jacobi-CG, which it already supports.

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

requirements.txt

@@ -1,6 +1,8 @@
 kicad-python>=0.7.0
 numpy
 scipy
-pyamg
+# no pyamg wheels for Linux aarch64, and KiCad installs wheels-only
+# (--only-binary): skip it there, the solver falls back to Jacobi-CG
+pyamg ; sys_platform != "linux" or platform_machine != "aarch64"
 matplotlib
 PySide6