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/app/tree.py

@@ -208,16 +208,27 @@ def place_memory(
     memory: models.Memory,
     branch: models.Branch | None = None,
 ) -> models.Branch:
-    """Attach a memory to the node that produced it.
+    """Attach a memory to the node it belongs to.
 
     `source_end` is the index of the last action the memory summarises, which is
-    that node's depth. A hand-written memory summarises nothing, so its depth
-    stays NULL and it belongs to the adventure rather than to a path.
+    that node's depth. A hand-written memory summarises nothing, so it takes the
+    head instead: **the story you were reading when you wrote it.**
+
+    That anchor is what makes a memory mean one thing (SP7). Before it, a
+    hand-written memory kept a NULL depth and "belonged to the adventure rather
+    than to a path" — which sounded harmless and meant it followed you onto
+    branches whose story it did not describe, because a NULL cannot be capped at
+    a fork. Every memory now sits at a coordinate, so "is this part of the story
+    I am reading?" has one answer for every row in the bank, and it is the same
+    answer the lineage already gives for nodes.
     """
     branch = branch or head_branch(db, adventure)
     memory.branch_id = branch.id
-    if memory.depth is None and memory.source_end is not None:
-        memory.depth = memory.source_end
+    if memory.depth is None:
+        memory.depth = (
+            memory.source_end if memory.source_end is not None
+            else adventure.head_depth
+        )
     return branch