feat(packaging): add a portable Windows build

[jsschwrz]

Copying this folder to another Windows machine has never worked: there is no
venv and no lockfile, so every dependency lives in a user-scoped Python 3.12
install outside the repo. Re-pointing a shortcut at main_hcs.py fails on the
first third-party import.

Add a build script that produces a relocatable bundle (~287 MB zipped) carrying
the interpreter, all 150 dependencies and the app, launched by a Squid.cmd that
pins the bundled interpreter and sets CWD to software/ -- both of which the app
requires and neither of which a bare shortcut provides.

The script copies the known-good 3.12 tree rather than resolving a fresh
environment, because the live dependency set has drifted from setup_22.04.sh
(which still pins numpy<2 and napari==0.5.4). requirements-windows.txt records
what actually runs; it sits at the repo root because software/.gitignore
ignores *.txt.

Also rewrite default_saving_path to a bare "Downloads" in the bundle. The guard
in control/_def.py strips slashes but not a "C:" drive prefix, so a foreign
absolute path is concatenated into garbage rather than falling back.

check_drivers.py verifies interpreter, dependencies, both camera libraries and
the serial port, which separates a driver problem from a Python problem on a
freshly copied bundle where pythonw.exe would otherwise fail silently.

The Daheng Galaxy SDK stays a target-machine prerequisite: gxipy loads
GxIAPI.dll by bare name off the system PATH and ships a kernel-mode USB driver,
so it cannot travel in a zip. The Toupcam DLL loads file-relative and does.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com