Rename is_change to is_presumed_change

The flag in _parse_outputs is set for two different strengths of
finding. For single sig our own key rebuilt the scriptPubKey, so the
output is provably ours. For multisig we have only shown that our key
sits in the committed script, and which wallet that script belongs to
is settled later, by the user's descriptor. Calling both "is_change"
gave the multisig case a certainty it does not have.

The process docstring is held to the same standard: it describes how
ownership is determined rather than how outputs are verified as change,
the verdict table reads "presumed change", and the multisig rebuild
bullet names the key-in-script step that the table row depends on.

No behavior change.

src/seedsigner/models/psbt_parser.py

@@ -331,16 +331,17 @@ class PSBTParser():
         OP_RETURN data, and totals the amounts for each. Note that self-transfer/receive
         outputs are also considered "change".
 
-        Most of the work here is determining which, if any, outputs are verifiably being
-        paid to our seed.
+        Most of the work here is sorting through the psbt's claims about which, if any,
+        outputs are paying a key that can be derived from our seed, and then doing all
+        possible independent verifications for the given output data.
         """
 
-        """****************** How outputs are verified as change ************************
-        Many outputs are obviously NOT change. An output is only considered possible
+        """********************* How output ownership is determined *********************
+        Many outputs are obviously NOT ours. An output is only considered possible
         change if its policy matches the inputs' policy "shape" (script type, plus m-of-n
         for multisig; see parse()); anything else is recorded as an external spend.
 
-        The psbt will usually annotate which key(s) own a change output (see embit's
+        The psbt will usually annotate which key(s) a change output pays (see embit's
         bip32_derivations and taproot_bip32_derivations), but this is just a claim
         supplied by the coordinator. These annotations are not authoritative. But such
         claims are significant; if our checks prove that the claim is false, we consider
@@ -354,7 +355,8 @@ class PSBTParser():
 
         We must build our own version of the scriptPubKey via:
           * single sig: derive a key from our seed using the claimed derivation path.
-          * multisig: hash the claimed witness_script or redeem_script.
+          * multisig: hash the claimed witness_script or redeem_script, then check that a
+            key derived from our seed is among that script's keys.
 
         That leaves us holding two independent answers about the same output: which key it
         commits to (our rebuild, matched against the scriptPubKey), and which key the psbt
@@ -362,12 +364,16 @@ class PSBTParser():
 
                                      | claims this seed     | doesn't claim this seed
             -------------------------+----------------------+-------------------------
-            commits to our key       | confirmed: is change | contradiction
+            commits to our key       | presumed change      | contradiction
             commits to another key   | contradiction        | presumed external spend
 
         If our two answers contradict each other, the psbt has been caught in a deception.
         We raise an exception and reject the psbt.
 
+        Multisig change can't be fully verified until later in the process, so we use
+        "presumed" to avoid conveying a false impression of certainty. Single sig carries
+        its own note later in this function about its guarantees.
+
         (note one exception: no taproot mismatch is rejected. A script tree tweaks our
         internal key, so an honest taproot change output fails to match too, and we cannot
         yet tell that apart from an output that claims our key but pays someone else. All
@@ -387,7 +393,7 @@ class PSBTParser():
 
         for i, out in enumerate(self.psbt.outputs):
             out_policy = PSBTParser._get_policy(out, vout[i].script_pubkey, self.psbt.xpubs, child_key_derivation_cache)
-            is_change = False
+            is_presumed_change = False
 
             # Is this output change? If this output's policy is superficially similar to
             # the spending wallet's policy (e.g. they're both 2-of-3 p2wsh), then it's a
@@ -497,9 +503,9 @@ class PSBTParser():
                             raise RuntimeError(f"Output {i} verified at a path it does not pay")
 
                         # We've now verified that the key we derived from our seed at the
-                        # claimed path is the key this output pays. This output is
-                        # provably ours.
-                        is_change = True
+                        # claimed path is the key this output pays. Despite the "presumed"
+                        # variable name, the output IS provably ours.
+                        is_presumed_change = True
 
                     elif multisig_script is not None:
                         if verified_derivation_path is None:
@@ -551,7 +557,7 @@ class PSBTParser():
                             # multisig that happens to include our seed. Final change
                             # verification can only happen if and when the user loads
                             # their "known-good" multisig descriptor.
-                            is_change = True
+                            is_presumed_change = True
 
                 elif verified_derivation_path is not None and self.policy["type"] != "p2tr":
                     # The psbt claims one of this seed's keys on this output, yet the
@@ -579,7 +585,7 @@ class PSBTParser():
                 # The data is written as: OP_RETURN + OP_PUSHDATA1 + len(payload) + payload
                 self.op_return_data = vout[i].script_pubkey.data[3:]
 
-            elif is_change:
+            elif is_presumed_change:
                 # Remember that "change" in this function is ANY output coming back to our
                 # seed, receive addresses included. It is up to the View layer to use the
                 # derivation path to determine if it should be displayed as change or