Add support for localpart:*@<domain> entries in SSH policy users.
When a user SSHes into a target, their email local-part becomes the
OS username (e.g. alice@example.com → OS user alice).
Type system (types.go):
- SSHUser.IsLocalpart() and ParseLocalpart() for validation
- SSHUsers.LocalpartEntries(), NormalUsers(), ContainsLocalpart()
- Enforces format: localpart:*@<domain> (wildcard-only)
- UserWildcard.Resolve for user:*@domain SSH source aliases
- acceptEnv passthrough for SSH rules
Compilation (filter.go):
- resolveLocalparts: pure function mapping users to local-parts
by email domain. No node walking, easy to test.
- groupSourcesByUser: single walk producing per-user principals
with sorted user IDs, and tagged principals separately.
- ipSetToPrincipals: shared helper replacing 6 inline copies.
- selfPrincipalsForNode: self-access using pre-computed byUser.
The approach separates data gathering from rule assembly. Localpart
rules are interleaved per source user to match Tailscale SaaS
first-match-wins ordering.
Updates #3049
0acf09bdd2