verify(P4): migrate-ssm-except-narrowing — 4-layer verify PASS + ship

VERIFY: structural — narrowed excepts; behavioral — 48 tests + CI PASS; security — non-ParameterNotFound errors surface; quality — 2 new + 2 updated tests.

---ci---
project: acdl
phase: 4
milestone: v1.16
status: complete
phase_role: execution
requirements:
  covered: [REQ-168]
  partial: []
---/ci---

scripts/migrate_ssm_paths.py

@@ -110,8 +110,18 @@ def copy_one_param(client, source_name: str, dest_name: str, force: bool = False
             return "skipped-equal"
         if not force:
             return "skipped-mismatch"
-    except Exception:  # ParameterNotFound → proceed to put
-        pass
+    except client.exceptions.ParameterNotFound:
+        pass  # target doesn't exist yet → proceed to put
+    except Exception as e:
+        # P4 (REQ-168): narrow the broad swallow — only ParameterNotFound
+        # is an expected "proceed to put" condition. Any other AWS error
+        # (auth, throttling, service) must surface, not be swallowed.
+        import sys
+        sys.stderr.write(
+            f"migrate_ssm_paths: get_parameter({dest_name}) failed: "
+            f"{type(e).__name__}: {e}\n"
+        )
+        raise
 
     put_kwargs = {
         "Name": dest_name,