feat(nats): per-license auth mechanism — agent user/password, scoped broker, generator (non-breaking)

Closes the open broker (anonymous publish to any tenant's corrosion.*).
Per-license isolation via NATS user/password + subject permissions:
each license -> user=license_id, password=HMAC-SHA256(license_id,
NATS_TOKEN_SECRET), scoped to corrosion.{license_id}.> + _INBOX. Backend
uses a privileged internal user.

- Agent (alpha.5): nats_user/nats_password config + env, user_and_password
  auth; falls back to token/anonymous (transition-safe)
- Backend: connects with NATS_INTERNAL_USER/PASSWORD when set, else anon
- scripts/generate-nats-auth.mjs: regenerates nats-auth.conf from the
  licenses table; NATS_AUTH_STAGE=open keeps a no_auth_user fallback
  (verify creds first), =enforce rejects anonymous
- committed nats-auth.conf is the SAFE OPEN default (no secrets); the
  host copy carries real users and is not committed
- compose: NATS_INTERNAL_USER/PASSWORD/NATS_TOKEN_SECRET, mount nats-auth.conf

Entirely non-breaking until secrets+config deployed; staged cutover next.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.4"
+version = "2.0.0-alpha.5"
 edition = "2021"
 description = "Corrosion Host Agent — multi-game ops runtime for self-hosted game servers"
 license = "UNLICENSED"