Docs: capture known durability gaps and validation guardrails

Author: sushildalavi

docs/reliability/known-gaps.md

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+# Known Gaps and Next Fix Targets
+
+- Redis drained with partial Postgres completion indicates durability boundary issues between stream acknowledgment and DB visibility.
+- Prefer validating any durability fix with clean-slate `down -v` benchmark runs.
+- Avoid publishing exactly-once guarantees; use at-least-once plus idempotent state convergence wording.