This adds a threat model for `setup-uv` so security scanners can use it
as a baseline in terms of what's in-, and out of scope.
The TM covers credential recipients, executable and cache boundaries,
and release authority. It treats checkout-selected interpreters, paths,
virtual environments, symlinks, and helpers as delegated project
authority unless they override an explicit workflow choice or cross an
independent cache, runner, remote, or publication boundary.
