Record the verified principal type on approver entries (schema v4)

An entry stored subject_id, assurance and evidence_ref but nothing about
what kind of principal bound the approval, and subject_id is a naming
convention rather than a verified claim. /entries is not restricted by
principal type — only /consume is — and the approval-engine-operator
client holds approval:approve, so a service can supply approver evidence
today. Whether it may is gate-house doctrine; that it is legible is ours.

Add entries.principal_type, populate it from the verified token, surface
it on the object and the audit evidence path (not the claim, which stays
least-disclosure), and migrate v3 stores leaving legacy rows null rather
than back-filling a claim nobody made.

Also corrects two statements in the requirements issued to
informed-decision: agent tokens are not barred from approval:approve, and
an empty assurance object is accepted rather than refused.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HyybaE7DUXrWYrhbnESCTe

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1275879@bnt-lap001
Assistant-Session: eb464208-f821-41b2-bc5a-a6c33d92a8ad

tests/test_cas.py

@@ -6,7 +6,7 @@ from pathlib import Path
 import pytest
 
 from approval_engine.errors import Conflict, Unprocessable
-from approval_engine.store import Engine
+from approval_engine.store import LATEST_SCHEMA_VERSION, Engine
 from tests.conftest import FROZEN, approve, binding, validity
 
 
@@ -174,8 +174,8 @@ def test_successor_inherits_the_pdp_path_declaration(engine):
     assert successor.pdp_path is True
 
 
-def test_v2_database_migrates_to_v3_preserving_approvals():
-    """Schema v3 (GH-DEC-2026-008) must not disturb approvals issued under v2."""
+def test_v2_database_migrates_forward_preserving_approvals():
+    """A v2 store must upgrade to the current schema without disturbing approvals."""
     with tempfile.TemporaryDirectory() as tmp:
         path = Path(tmp) / "v2.sqlite"
         eng = Engine(path, clock=lambda: FROZEN)
@@ -189,7 +189,7 @@ def test_v2_database_migrates_to_v3_preserving_approvals():
 
         upgraded = Engine(path, clock=lambda: FROZEN)
         version = int(upgraded._conn().execute("PRAGMA user_version").fetchone()[0])
-        assert version == 3
+        assert version == LATEST_SCHEMA_VERSION
         survivor = upgraded.get(obj.id)
         assert survivor.status == "approved"
         assert survivor.pdp_digest == "sha256:" + "12" * 32
@@ -197,4 +197,28 @@ def test_v2_database_migrates_to_v3_preserving_approvals():
         # a digest that happens to be present
         assert survivor.pdp_path is False
         assert upgraded.claim(obj.id)["binding"]["pdp_path"] is False
+        # v4: a legacy entry carries no verified statement about the kind of
+        # principal that bound it, and must not be back-filled into one
+        assert survivor.entries[0]["principal_type"] is None
+        upgraded.close()
+
+
+def test_v3_entries_migrate_to_v4_without_inventing_a_principal_type():
+    """The v4 column is added; pre-v4 approver entries stay unclassified."""
+    with tempfile.TemporaryDirectory() as tmp:
+        path = Path(tmp) / "v3.sqlite"
+        eng = Engine(path, clock=lambda: FROZEN)
+        obj = eng.create(binding(), validity())
+        eng.add_entry(obj.id, "user:alice", principal_type="human")
+        eng._conn().execute("ALTER TABLE entries DROP COLUMN principal_type")
+        eng._conn().execute("PRAGMA user_version=3")
+        eng._conn().commit()
+        eng.close()
+
+        upgraded = Engine(path, clock=lambda: FROZEN)
+        assert int(upgraded._conn().execute("PRAGMA user_version").fetchone()[0]) == 4
+        survivor = upgraded.get(obj.id)
+        assert survivor.status == "approved"
+        assert survivor.entries[0]["subject_id"] == "user:alice"
+        assert survivor.entries[0]["principal_type"] is None
         upgraded.close()