Document clean Windows alpha installation verification

[HabsaTheDog]

Goal: turn the release policy into a reproducible Windows x64 verification checklist for the first alpha.

Scope:

• use only an artifact produced by the Alpha desktop artifacts workflow;
• verify SHA256SUMS before installation;
• record SmartScreen/signature behavior, installer/uninstaller behavior, Study Buddy product identity, dedicated state paths, and loopback-only backend binding;
• use synthetic data and do not connect a real student account;
• add the resulting checklist under docs and link it from docs/releasing.md.

Acceptance:

• another contributor can repeat every step on a clean Windows machine;
• expected evidence and failure criteria are explicit;
• no private screenshots, usernames, paths, or credentials are committed.