[AZ-447] Close testability refactor run; advance to Step 5

- Archive Batch 2 task specs (AZ-448, AZ-449, AZ-453) to
  _docs/02_tasks/done/.
- Write testability_changes_summary.md (refactor Phase 4.5; user-acked
  via autodev existing-code Step 4 gate).
- Write FINAL_report.md closing the 01-testability-refactoring run.
- Advance autodev state pointer to Step 5 (Decompose Tests).

Refactor Phases 5/6/7 are no-ops for testability runs (no tests exist
yet; doc updates are deferred to autodev Step 13). Verification axis
for this run is the static-check matrix recorded in
testability_changes_summary.md § Verification snapshot.

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

_docs/_autodev_state.md

@@ -2,13 +2,13 @@
 
 ## Current Step
 flow: existing-code
-step: 4
-name: Code Testability Revision
-status: in_progress
+step: 5
+name: Decompose Tests
+status: not_started
 sub_step:
-  phase: 4
-  name: refactor-phase-4-implement
-  detail: "batch 2 of 2 — AZ-448, AZ-449, AZ-453"
+  phase: 0
+  name: awaiting-invocation
+  detail: ""
 retry_count: 0
 cycle: 1
 step_4_5_glossary_vision: confirmed