v0.3.6 — Windows write-permission fix

Critical bugfix release. On Windows, running 'alphadynamics' from a directory you don't have write access to (e.g. 'C:') would let the prediction complete in full, then crash with PermissionError when saving — losing all the computed trajectories.

Fix: pre-flight write check happens BEFORE the model runs, with auto-fallback to ~/Documents/ (or tempfile.gettempdir()) and a friendly notice telling you where the file actually landed. Same protection for both .npz and Ramachandran plot files.

Affects: anyone running on Windows from C:\ root or any read-only directory.
Linux/macOS users were also vulnerable when running from /sys, /, or other restricted paths.

Headline (unchanged): mean canonical Ramachandran JSD 0.196 vs Microsoft Timewarp 0.468 on the 4AA benchmark, ~123K parameters.

Bug found by Krzysztof Gwozdz on Windows 11 / Python 3.13 / cmd.exe.