CRYPTO-0: model SHA-256 + HMAC-SHA-256 in .dag; switch extdeps.crypto.mac; delete the hmac primitives

Cargo.toml

@@ -44,7 +44,6 @@ sha2 = "0.10"
 # repo-wide crypto upgrade. RustCrypto provides Hmac<Sha256> together with verify_slice, which
 # compares in constant time -- the operation gunbc.auth.approval_capability requires and the reason
 # no accessor in extdeps.crypto.mac yields a computed tag for a caller to compare itself.
-hmac = "0.12"
 hex = "0.4"
 glob = "0.3"
 ureq = "2"

dag/extdeps/cloud/gcp/secret_manager.dag

@@ -1,7 +1,8 @@
 module extdeps.cloud.gcp.secret_manager
 
-import std.encoding { Standard, base64_decode, base64_encode, utf8_decode_bytes }
-import std.bytes { bytes_octets, octets_bytes, utf8_encode_bytes }
+import std.machine_word { QualifiedOctetsReady, QualifiedOctetsRefused }
+import std.encoding { Standard, base64_decode, base64_encode, base64_octets, utf8_decode_bytes }
+import std.bytes { bytes_octets, octets_bytes, utf8_encode_bytes, pure_dag_seam_unreachable_string }
 import extdeps.cloud.gcp.iam { GcpBinding, GcpPolicy }
 import extdeps.external_authority { ExternalAuthority }
 import extdeps.uri { Uri, Https }
@@ -199,11 +200,22 @@ fn sm_resolved_version_identity_is_well_formed(name: String) -> Bool {
   }
 }
 
+// base64_encode gained a refusal channel when NUMERIC-BIT-0 (gunbc#11627) routed its packing
+// through std.machine_word: it refuses an octet outside the byte range, where it previously
+// multiplied one into a plausible wrong string. THE ARM IS UNREACHABLE HERE, and that is a
+// statement about this call and not about base64_encode: the octets are the UTF-8 encoding of a
+// String, so every member is a byte by construction and no in-range input can reach the refusal.
+// An unreachable arm still has to answer, and "" is a valid base64 encoding -- of the empty
+// credential -- so returning it would conflate a refusal with a real answer at a secret-provisioning
+// boundary. std.bytes' divergent seam is the corpus's declared idiom for an arm that cannot be
+// reached; it diverges rather than fabricating, which is the loud half of DESIGN section 5.
+// This function's signature is deliberately unchanged: threading an Optional to the two actuators
+// that call it would put a refusal channel where no refusal can arise.
 fn encode_sm_access_version_payload_wire(credential: Secret) -> String {
-  base64_encode(
-    octets: bytes_octets(b: utf8_encode_bytes(s: credential as String)),
-    variant: Standard
-  )
+  match base64_octets(values: bytes_octets(b: utf8_encode_bytes(s: credential as String))) {
+    QualifiedOctetsReady { octets: qualified } => base64_encode(octets: qualified, variant: Standard)
+    QualifiedOctetsRefused { cause: _ } => pure_dag_seam_unreachable_string()
+  }
 }
 
 fn decode_sm_access_version_payload_wire(data_b64: String) -> SmAccessVersionPayloadDecodeOutcome {