docs(swarm): add the swarm-nats-auth package readme

Every other crate has one and Cargo.toml already named it. Follows the
sibling shape: what it is, why it is a crate rather than more config, the
invariant that is easy to break (issuer_account must be absent, and
Token::new_user reintroduces it), and what the tests do NOT cover - the
introspection endpoint under test is a stub, so nothing here says anything
about the real authelia integration.

swarm-nats-auth/README.md

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+# swarm-nats-auth
+
+The auth-callout responder for the swarm's NATS queue — the half that lets
+the server say *yes*.
+
+`nix/host-modules/swarm-nats.nix` configures `nats-server` with an
+`auth_callout` block. **That block with no responder is the fail-closed
+state**, and it is the measured one rather than the obvious one: on the pinned
+nats-server, both `authorization { }` and `authorization { users: [] }` answer
+PONG to an anonymous client, while an `auth_callout` block sets
+`auth_required` and refuses every client no responder has approved. So the
+module ships the final config from the start and this crate only adds the
+ability to approve. No interim hole is ever opened.
+
+## Why a crate and not more config
+
+The reply is a **signed NATS user JWT**. Signing needs the account nkey seed
+and the JWT framing, which is program work — which is why the container and
+its config landed first and this arrived separately, rather than the pair
+being one change.
+
+## The invariant that is easy to break
+
+⛔ **`issuer_account` must be absent from the issued user token.** It is an
+operator-mode field. The module renders *server-config* mode (`accounts
+{ AUTH, APP }`, no operator), where its mere presence makes the server refuse
+the client — `Error non operator mode account "AUTH": attempted to use
+issuer_account` — while the responder cheerfully reports `granted=true`. The
+account is named by the claims' `aud` instead.
+
+`nats_jwt::Token::new_user` always sets it, so reaching for that constructor
+reintroduces the bug. `nats-jwt` is a **dev-dependency**: it cannot express
+`aud` on either token, and its role here is as the encoder's *test oracle*,
+not part of the path that runs.
+
+## Rules the code follows
+
+- **A denial is a signed response carrying an error, never silence.** The
+  server cannot tell an absent responder from a refusing one, so staying quiet
+  turns every rejection into a timeout and hides an outage inside what looks
+  like ordinary denials.
+- **Anything that is not an explicit `{"active": true}` denies** — including
+  an introspection call that could not be *made*. The failure modes of an HTTP
+  call are exactly the conditions under which an attacker would most like this
+  to fall open.
+- **Every credential is a path, never a value.** A value in nix config lands
+  in the world-readable store; a value in `argv` is readable via
+  `/proc/<pid>/cmdline`, which is `0444`. Paths are not secrets, so passing
+  them as flags is fine.
+- **The introspection timeout is pinned below the server's `authorization.
+  timeout`**, with a test asserting the relation — a responder that answers
+  after the server gave up is indistinguishable from one that never answered.
+
+## What the tests do and do not cover
+
+Unit tests cover the JWT framing, including byte-equality against `nats-jwt`
+on the one shape that crate models. They are **not** sufficient on their own:
+this crate's shape is decided by a server that parses what it emits, so the
+change was also driven against a real `nats-server` — every refusal repeated
+*with the responder live*, because a responder that says yes to everyone
+passes "a client can connect" perfectly. That harness lives outside this repo;
+the PR that added this crate links it.
+
+⚠️ **Its introspection endpoint is a stub.** Those runs prove this crate's own
+behaviour and nothing about the real authelia integration, which needs a
+deployment.