path: add `homedir(username)` method for looking up other users' home directories
On Unix, uses `getpwnam_r` for thread-safe passwd lookup to find
the home directory for a given username. On Windows, resolves the
username to a SID via `LookupAccountNameW` and reads the profile
path from the registry; falls back to inferring the profile directory
from the current user's home if the registry lookup fails.
Returns `nothing` if the user does not exist.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>