docs(architecture): correct the fingerprint base64 alphabet wording

The format contract (ADR-004 decision 1) is SHA256: + unpadded
standard base64 — not url-safe. The ssh-key re-export and ssh-keygen
both emit the +/ alphabet (visible in the recorded fingerprints); the
'url-safe' wording in ADR-004/auth.md/plans/implementation.md and the
POC-1 §1 finding was a misnomer. Wording corrected; the parity
contract itself unchanged on both sides.

docs/architecture/auth.md

@@ -23,10 +23,12 @@ verifies a signature; its auth surface is *dispositional* (ADR-004).
    0.65.0 shape — the key is the `.public_key` field, and the client's
    signature `.algorithm` is exposed for optional hardening).
 2. alkssh computes the fingerprint via the `ssh-key` re-export
-   (`HashAlg::Sha256`) — `SHA256:` + unpadded url-safe base64. The
-   string format is a **contract invariant** (ADR-004): parity with
-   assembly-registered values is proven against independent issuers
-   (`ssh-keygen -lf` — POC-1 §1).
+   (`HashAlg::Sha256`) — `SHA256:` + unpadded base64 over the
+   standard alphabet (`ssh-keygen -lf` byte-format; not the
+   url-safe alphabet). The string format is a **contract invariant**
+   (ADR-004): parity with assembly-registered values is proven
+   against independent issuers (`ssh-keygen -lf` — POC-1 §1, pinned
+   by the committed vectors in `tests/vectors/`).
 3. `resolve_from_fingerprint` → `Some(identity)` → accept; `None` →
    reject — fail-closed, and the resolved-identity-`None` path never
    creates a "weakly authorized" session.

docs/architecture/decisions/004-auth-dispositions.md

@@ -37,7 +37,9 @@ twice: which methods exist, and what unauthenticated attempts receive.
 
 1. **`auth_publickey` is implemented — the only method.** Handler:
    fingerprint (`ssh-key` re-export, `HashAlg::Sha256`,
-   `SHA256:` + unpadded url-safe base64) →
+   `SHA256:` + unpadded base64 over the standard alphabet —
+   `+/`, not the url-safe alphabet; byte-identical to
+   `ssh-keygen -lf`) →
    `resolve_from_fingerprint` → `Some` → accept with the resolved
    identity; `None` → reject. The fingerprint *string format* is a
    contract invariant: registration strings from any standard issuer

docs/plans/implementation.md

@@ -101,7 +101,7 @@ test. The alkssh analog of alkgit's seam wave.
   pins (hard-decline on mismatch) and case-2 assembly policies are the
   implementor's; the crate ships the fail-closed default (reject
   unknown) and is otherwise policy-blind — no accept-all built-in.
-- **Fingerprint** (ADR-004) — the `SHA256:` + unpadded url-safe base64
+- **Fingerprint** (ADR-004) — the `SHA256:` + unpadded base64
   computation via the `ssh-key` re-export (`HashAlg::Sha256`); the
   string format is a contract invariant, proven against independent
   issuers (`ssh-keygen -lf` vectors, the POC-1 §1 checkpoint shape).

docs/research/poc-1-auth-bridge.md

@@ -32,8 +32,11 @@ the OQ seeds this POC produced.
 ### 1. Fingerprint-format parity — confirmed, format is exactly the registered string
 
 `public_key.fingerprint(HashAlg::Sha256).to_string()` (via the
-`ssh-key` re-export) yields `SHA256:<url-safe base64, unpadded>` —
-byte-identical to `ssh-keygen -lf` on the same public key. The
+`ssh-key` re-export) yields `SHA256:<unpadded base64, standard
+alphabet>` — byte-identical to `ssh-keygen -lf` on the same public key. (An
+earlier phrasing of this section called the encoding "url-safe"; the
+observed strings and OpenSSH's own format use the standard alphabet —
+unpadded standard base64 is the invariant.) The
 `resolve_from_fingerprint` string compare works with values that were
 registered from OpenSSH tooling output. Parity was additionally
 observed against the vanilla client's own host-key check