Add head/draft/autosave hierarchy to Node

Introduces autosave_id as a third, unversioned layer above draft/head,
with lock_for_editing!, autosave!, and save_draft! as the new entry
points. Also fixes a real bug in wipe_draft!: its "no draft" branch
unconditionally released the lock, which was safe when "no draft" only
ever meant "nothing is happening" — no longer true now that a lock can
exist with only an autosave beneath it. lock_for_editing! deliberately
does not call wipe_draft! at all, for the same reason: an intruder
calling it while a lock was genuinely held would otherwise silently
steal it via wipe_draft!'s own unlock side effect, caught by the new
two-user lock test.

1 files changed, 5 insertions, 0 deletions

diff --git a/db/migrate/20260708095943_add_autosave_id_to_nodes.rb b/db/migrate/20260708095943_add_autosave_id_to_nodes.rb
new file mode 100644
index 0000000..eedcc00
--- /dev/null
+++ b/db/migrate/20260708095943_add_autosave_id_to_nodes.rb
@@ -0,0 +1,5 @@
+class AddAutosaveIdToNodes < ActiveRecord::Migration[8.1]
+  def change
+    add_column :nodes, :autosave_id, :integer
+  end
+end