feat(authz): operator signed-op verifier + durable nonce store (slice 2, v0.2.0)

internal/authz: production form of the Phase-4 SSHSIG signing primitive.

- Verifier.New/Verify with the LOCKED pipeline (namespace → allow-list by key
  material → crypto over RAW bytes → target → time → nonce LAST); each post-crypto
  stage rejects even with a valid sig; an invalid sig never burns a nonce.
- SSHSIG framing via x/crypto/ssh (no hand-rolled crypto); key-type-agnostic
  (ed25519 / sk-ssh-ed25519 / rsa / ecdsa via pub.Verify). Fixed namespace
  felhom-op-v1. Typed errors. OpBlob (fixed host_id/guest_id tags) + VerifiedOp.
- NonceStore: MemoryNonceStore + durable crash-safe FileNonceStore (fsync'd append
  log, replay-on-open, compaction, expiry-only pruning; survives restart).
- config.AuthzConfig (nonce path + pinned operational/recovery signer keys).
- Tests (14): real ssh-keygen fixture, per-stage rejection, nonce-not-burned,
  replay, persistence-across-restart, synthetic sk, byte-exactness.

Dep: golang.org/x/crypto v0.52.0 (declares go 1.25 — the Phase-4 doc's "Go 1.24.4 /
x/crypto v0.52.0" pairing doesn't build; build server upgraded to go1.26.0,
backward-compatible). Version 0.1.0 -> 0.2.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/authz/testdata/op_blob.json

@@ -0,0 +1 @@
+{"expires_at":"2026-06-09T00:00:00Z","issued_at":"2026-06-08T00:00:00Z","key_id":"felhom-op-1","nonce":"a1b2c3d4e5f60718293a4b5c6d7e8f90","op":"guest_destroy","params":{"purge":true},"target":{"guest_id":"9001","host_id":"demo-felhom"}}

internal/authz/testdata/op_blob.sig

@@ -0,0 +1,6 @@
+-----BEGIN SSH SIGNATURE-----
+U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgNXOOuMvD3Fh9MJYspBRWLXyQAd
+WVeBICspeB9eL1xfIAAAAMZmVsaG9tLW9wLXYxAAAAAAAAAAZzaGE1MTIAAABTAAAAC3Nz
+aC1lZDI1NTE5AAAAQG+bj+GNodNw7cfGYg3HWTDyJiu3g/5Aez1xlZQ540JGUIG9FV7vv8
+wrgN0r+rNh+ytEAM6UTOyI7g3LOjuVJgY=
+-----END SSH SIGNATURE-----

internal/authz/testdata/operator.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDVzjrjLw9xYfTCWLKQUVi18kAHVlXgSArKXgfXi9cXy felhom-op-1