Give every memory a node, and show only the ones on your path

A hand-written memory used to carry a NULL depth, described in the model as
"belongs to the adventure rather than to a path". That sounds harmless and
is not: a NULL is a coordinate no fork can cap, so a note typed on one line
followed the reader onto branches whose story it never described. It takes
the head now — the story you were reading when you wrote it — and obeys
exactly the rule a summarised memory obeys.

The unanchored escape clause in lineage.Path.clause existed for that single
case and is deleted rather than left unused. Its docstring argued that a
capped depth would drop a typed memory the moment its branch stopped being
the newest entry; anchoring answers the same worry better, because the
memory is not exempt from the path, it is on one.

The drawer now shows the path being read and nothing else, filtered by the
clause retrieval itself uses, so the bank you can see is the bank the model
can see. Nothing is stranded: a memory lives on a branch, switching to that
branch shows it, and deleting the branch deletes it. Pinning decides order,
the path decides existence.

Migration 62 lands existing NULL-depth memories at depth 0 of their branch
rather than at the tip. 0 is at or before every fork point, so every memory
stays visible from exactly the paths it is visible from today — nobody's
bank loses a row on deploy. The tip is the tidier-sounding choice and would
have emptied them out of every branch forked earlier than they were typed.

This supersedes the on_path flag and the "another branch" badge from
earlier today; anchoring makes them redundant, and they are removed.

Four tests changed because they asserted the old contract, not because
they broke. The one worth reading is the pair replacing
test_a_hand_written_memory_is_not_lost_at_the_first_fork: typed on shared
trunk it still survives a fork, and typed on ground the fork never
travelled it no longer follows you.

402 tests. Verified on tools/branch_fixture.py: each branch's drawer holds
its own memory and not the other's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015H5qiyiR7gtFQaoDphHZ3g

backend/tests/test_tree_migration.py

@@ -276,10 +276,12 @@ def test_memories_attach_to_the_node_they_summarised(pre_tree):
         assert depth == source_end, "the memory hangs off the last action it covered"
         assert branch_id is not None
 
-    # A hand-written memory has no node: it gets a branch, but no depth, which
-    # SP3 reads as belonging to the adventure rather than to a path.
+    # A hand-written memory summarised no node, so SP7's migration 62 lands it
+    # at depth 0 of its branch. 0 is at or before every fork point, so it stays
+    # visible from exactly the paths it was visible from before — anchoring
+    # takes nothing out of anybody's existing bank.
     manual = rows("SELECT depth, branch_id FROM memories WHERE source_end IS NULL")
-    assert manual and all(depth is None and branch is not None for depth, branch in manual)
+    assert manual and all(depth == 0 and branch is not None for depth, branch in manual)
 
 
 def test_the_cursors_become_the_nodes_they_named(pre_tree):
@@ -439,7 +441,13 @@ def test_a_blank_adventure_has_a_branch_before_anything_is_played(client):
         db.close()
 
 
-def test_a_hand_written_memory_gets_a_branch_but_no_depth(client):
+def test_a_hand_written_memory_is_anchored_at_the_head(client):
+    """SP7: nothing carries a NULL depth any more.
+
+    On an adventure with no story yet the head is NO_DEPTH (-1), which reads as
+    "before the first node" and so is in range of every branch — right for a
+    note written before anything has happened.
+    """
     adventure_id = client.post("/api/adventures", json={}).json()["id"]
 
     created = client.post(
@@ -450,8 +458,9 @@ def test_a_hand_written_memory_gets_a_branch_but_no_depth(client):
     db = SessionLocal()
     try:
         memory = db.query(models.Memory).filter_by(adventure_id=adventure_id).one()
+        adventure = db.get(models.Adventure, adventure_id)
         assert memory.branch_id is not None
-        assert memory.depth is None
+        assert memory.depth == adventure.head_depth == -1
     finally:
         db.close()