fix(plugin): join plugin-opened undo scopes in XRecord writes

The internal per-call push_undo fractured plugin-composed operations:
history has no nested scopes, so each push finished the plugin's
still-pending scope and opened its own — an operation of several
writes became several undo steps and the plugin's label was lost
(measured: 3 creates + update + delete in one scope = 5 steps).

Follow the entity-write contract instead: a mutation inside a
plugin-opened push_undo scope joins it, so the whole operation is a
single undo step under the plugin's label; a scope-less mutation opens
and finishes its own scope, keeping one undo step per call; refusals
record no undo step in either regime. Document the contract on the
trait methods and the host docs; the composition experiment becomes
the permanent xrecord_writes_join_a_plugin_initiated_undo_scope test.

docs/plugin-host-model.md

@@ -95,10 +95,13 @@ dictionaries are created on create only, so the first call seeds a plugin's
 namespace. The payload is the codec's typed `XRecordEntry` pairs — the host puts
 no opinion on the content. Names and path fragments are symbol-validated, and
 dictionary addressing follows the codec's case-insensitive lookup. Create never
-overwrites and update never creates; each refusal is decided before the undo step
-is recorded, so a failure changes nothing, and each change is one undo step whose
-undo restores the objects map byte-exactly, the auto-created dictionaries
-included. `list_xrecords` reports only the keys that address XRecords, in the
+overwrites and update never creates; each refusal is decided before the undo
+scope is touched, so a failure changes nothing, and each change joins a
+plugin-opened `push_undo` scope — the entity-write contract — so an operation
+composed of several writes (entities and XRecords) is a single undo step; with
+no scope open a change is one undo step of its own, whose undo restores the
+objects map byte-exactly, the auto-created dictionaries included.
+`list_xrecords` reports only the keys that address XRecords, in the
 dictionary's own order, so audits need no prior knowledge. Not yet mapped to
 Python.