[AZ-500] Cycle 4 Step 17: retrospective + close cycle

Adds retro_2026-05-12_cycle4.md, structure_2026-05-12_cycle4.md, and
the deploy_cycle4.md report that was dropped from the Steps 12-15
sync commit. Appends 3 new lessons to LESSONS.md (12/15 ring buffer)
on transitive major-version bumps, exposed pre-existing bugs, and
single-task-cycle metric framing. State advances to cycle 5 / step 9
(awaiting next New Task invocation).

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -2,14 +2,14 @@
 
 ## Current Step
 flow: existing-code
-step: 16
-name: Deploy
-status: in_progress
+step: 9
+name: New Task
+status: not_started
 sub_step:
-  phase: 1
-  name: commit-cycle4-sync
+  phase: 0
+  name: awaiting-invocation
   detail: ""
 retry_count: 0
-cycle: 4
+cycle: 5
 tracker: jira
 auto_push: true