Split hybrid public keys into separate wire fields

The packed 1216-byte hybrid public had no consumer that wanted it
whole -- everything unpacked immediately. Message layouts and the
credential record now carry peer_m | peer_x as explicit fields; _PUB
and the unpack-at-use idiom are gone. credential_id values change with
the record layout. Step comments in the exchange methods explain what
each solve/challenge proves.

quantumpipe/sansio.py

@@ -38,7 +38,7 @@ def _describe(obj) -> str:
 # given to both parties out of band and stored as a bare 32-byte store
 # entry. The first successful registration replaces it with the composite.
 
-_CREDENTIAL = struct.Struct("32s32s1216s4s?")  # idroot | shared | peer pub | sas | vf
+_CREDENTIAL = struct.Struct("32s32s1184s32s4s?")  # idroot|shared|peer_m|peer_x|sas|vf
 
 
 def generate_registration_secret() -> bytes:
@@ -240,10 +240,12 @@ class Session:
             (
                 self._idroot,
                 self._shared,
-                self._peer_pub,
+                peer_m,
+                peer_x,
                 self._sas,
                 self._credential_verified,
             ) = _CREDENTIAL.unpack(cred)
+            self._peer_pub = (peer_m, peer_x)
             self._registering = False
             hs = Resumption(self._shared, self._idroot, self._peer_pub, self._ad)
         self._exchange = hs
@@ -263,7 +265,7 @@ class Session:
         # persists the store itself if it wants durability
         self._credential_verified = verified
         self._stored = _CREDENTIAL.pack(
-            self._idroot, self._shared, self._peer_pub, self._sas, verified
+            self._idroot, self._shared, *self._peer_pub, self._sas, verified
         )
         self._store[self._key_id] = self._stored