Docs: add failure matrix for convergence triage

Author: sushildalavi

docs/reliability/failure-matrix.md

@@ -0,0 +1,8 @@
+# Failure Matrix
+
+| Signal | Expected Value | Interpretation |
+|---|---:|---|
+| Redis group lag | 0 | Stream is fully drained |
+| Redis pending | 0 | No orphaned PEL entries remain |
+| Postgres completed | = total seeded | End-to-end durable convergence |
+| Postgres failed/terminal | 0 | No budget-exhausted work items |