feat(identity): offline Ed25519 verifier — issuer-free identity for the auth seam - #234

…he auth seam

The first verifier for the seam merged in #232, and deliberately not an IdP one.

WHY NOT OIDC FIRST. An OIDC verifier checks a token offline, but *obtaining* one needs the internet and the
IdP up — and shortening token lifetimes to tighten revocation makes that dependency worse, so the two cannot
be tuned together. This fleet is tailnet-scoped and dependency-free on purpose (tailnet-only binds,
deny-by-default ACLs, --unshare-net on confined steps, "tailnet = reachability, not authorization"); an
external IdP would be the first service the governance kernel needs UP to function, and a third party could
lock the operator out of approving actions on their own fleet. This has no issuer, no JWKS, and no network in
either direction.

THE SHAPE. The bearer is a self-contained short-lived ASSERTION: a DSSE envelope — the same infra.cwp.sign
surface that already signs grants and exod results, no new crypto — over
{pub, iat, exp, nonce}, base64url'd to fit an Authorization header. The assertion carries its own public key,
the signature is checked against THAT key, and the verifier returns sign.keyid(pub) = "ed25519:<16 hex>" as
the subject.

That is not circular, and the distinction is the whole design: anyone can mint a valid assertion with a key
they generated and it WILL verify. What they cannot do is make it resolve to a principal — resolve_principal
matches a subject only against one a registry entry DECLARES. So the signature proves possession of a key;
the mounted principals.json decides whether that key is anybody. Revocation is deleting a line from a file
already bind-mounted read-only into every node: instant, offline, no expiry window to wait out, no issuer to
consult.

Fail-closed everywhere, 21 tests, each asserting a refusal:
- a VALID assertion from an undeclared key resolves to nobody (the property that matters most);
- expired, future-dated (a lying clock cannot post-date its way to a long credential), and a hand-rolled
  assertion claiming a year-long TTL — the verifier does not trust the minter's clamp;
- tampered payload (swapping the embedded pubkey breaks the signature — it is self-verifying);
- a pubkey that is not 32 bytes; malformed/undecodable bearers; a bearer SECRET under this scheme;
- REPLAY: a spent nonce is refused inside its own validity window, and when the bounded cache is full of
  LIVE nonces it refuses rather than evicting one and permitting a replay.

Mutation-verified: disabling the replay check fails 2 tests; disabling signature verification fails the
tamper test. The verifier never raises and never reveals WHY it refused — a caller that could tell "bad
signature" from "expired" from "replayed" is an oracle, and govd answers 401 either way.

Governed gate (cws-pm, 16 validators incl. grant forge/replay/expiry/wrong-run/capability): 16/16
(5d8cb29162d34fa4). Local: ed25519_auth, principals, govd, fleetd, acl all green.

NEXT (not this PR): the OIDC overlay sits ABOVE this, never beside it — authenticate to an IdP, receive a
short-lived Ed25519 certificate signed by an operator CA, verified offline against a pinned CA pubkey. The
SSH-CA / Sigstore pattern. That moves the online dependency from every claim to periodic enrollment, and
degrades correctly: without the overlay this half is a complete working system, enrolled by hand.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>