feat(felis): add felis nano — Yggdrasil hasJoined multiplexer without a control plane

`felis nano` serves the vanilla sessionserver protocol
(GET /session/minecraft/hasJoined) as a federating multiplexer over
Mojang plus any number of third-party Yggdrasil roots, with no k3s,
Postgres, or panel — a MultiLogin-style auth front-end delivered as a
subcommand of the single felis binary rather than a separate build.

- config.LoadNano reads only [[auth_source]] blocks; it skips the
  database.url / root_domain / archive requirements the full server
  needs. Zero sources is valid (Mojang-only).
- Mojang is prepended in code (Identity:true), never from config, so it
  is always the sole identity root. Third-party profiles are rewritten
  to canonical = UUIDv3(felisAuthNS, tag+":"+nativeID).
- validateAuthSources rejects unknown keys, duplicate tags, and
  scheme-less URLs — a malformed nano config fails loud at load.
- Reuses api.HasJoinedHandler with a stub Repo (no blacklist backend);
  a rejected login is a 204, matching the vanilla sessionserver.
- nano.go binds the -listen flag and ignores [server] listen in config.

Verified on WSL (go1.26.4): go build/vet/test ./... green; a runtime
smoke against the template config returns 204 on a miss and logs
"Mojang + 0 third-party source(s)"; a duplicate-tag config exits
non-zero citing "unique".

cmd/felis/run.go

@@ -14,6 +14,7 @@ Commands:
   migrate up        Apply embedded database migrations under an advisory lock
   operator          Run the MinecraftServer controller-manager
   api               Run the felis-api HTTP server
+  nano              Run the Felis-nano hasJoined multiplexer (multi-Yggdrasil, no control plane)
   reaper            Run the world reaper / backup batch
   restore           Extract a world archive into a world volume (internal Job entrypoint)
   backup            Archive a world into the backup store and record it (internal Job entrypoint)
@@ -40,6 +41,8 @@ func run(args []string, stdout, stderr io.Writer) int {
 		return cmdOperator(rest, stdout, stderr)
 	case "api":
 		return cmdAPI(rest, stdout, stderr)
+	case "nano":
+		return cmdNano(rest, stdout, stderr)
 	case "reaper":
 		return cmdReaper(rest, stdout, stderr)
 	case "restore":