Codex regression-gate finding: since Session.save() no longer auto-clears the
truncation_watermark, the unconditional 'timestamp > watermark' skip in
merge_session_messages_append_only became a permanent ceiling — a genuine future
state.db-only row (recovery/compaction, missed by the sidecar) would be silently
dropped from /api/session and model-context reconstruction forever. Only apply the
above-watermark skip while the sidecar has NOT advanced past the watermark. Preserves
the #2914 deleted-tail filtering (revert-verified). Adds 2 regression tests.
