DC-EPOCH-22

BOUNDARY-ALIGNED-MARK-CAPTURE. The live epoch-boundary stake mark is captured ONLY from the durable reduced checkpoint materialized at the EXACT selected-chain boundary point -- the last durable block of the closing epoch (s_prev) -- never at a later catch-up tip and never via a per-block stake scan. The capture is durably bound as a BoundaryMark keyed by the canonical boundary chain point (slot, hash) and the checkpoint lineage at that point, persisted BEFORE the accumulator boundary transition consumes it. The transition consumes a mark only when the binding is present and its point + lineage match the canonical chain; a reorg that removes or replaces the boundary point INVALIDATES the mark and forces deterministic rematerialization (reset-to-seed + replay) -- a mark is NEVER reused on an epoch-number match alone. This protects the boundary transition's INPUT (DC-EPOCH-21 governs the transition's OUTPUT given a correct mark): a mark read at the catch-up tip is byte-wrong (the checkpoint is already past the boundary), and even at the steady-state tip it wrongly includes the first block of the new epoch (SNAP captures the end-of-epoch stake, BEFORE that block). A co-advancer segments the reduced-checkpoint + accumulator advance at each boundary (idempotent-resume, byte-identical to advance-to-tip), captures sum_base_credential_stake() at the boundary point, durably binds the BoundaryMark before the cross, then crosses the accumulator over the boundary block with the bound mark; finally it advances the checkpoint the rest of the way to the durable tip (EVIEW currency preserved). OBSERVE-ONLY in S3: a capture/cross fault STALLS the accumulator (it stays at s_prev), never halting the proven follow; the EVIEW checkpoint still reaches tip fail-closed. Enforced PROGRESSIVELY: #2b-i wires the accumulator boundary-cross entry point (this sub-commit, cross_accumulator_over_boundary_block, the ONLY place the S2 mark-exclusion is lifted); the durable BoundaryMark witness + the point/lineage validation (#2b-ii), the node_lifecycle co-advancer segment->capture->bind->cross->tip (#2b-iii), the DC-EPOCH-22 CI guard, and the CE-3c live venue crossing ALL LANDED (#2b-i/ii/iii 8d047dee/ee33cc4c/8232fe73 + the CI guard). CE-3c PROVEN LIVE 2026-06-29: two preview crossings (1338->1339 seam + 1339->1340 native), mark captured at the boundary point s_prev while the catch-up tip ran 70,655 slots ahead (proof ~/.cardano-ce3c-proof/). Status stays declared: the mechanism is live-proven but OBSERVE-ONLY in S3 -- the flip to enforced is the cluster-close event with the accumulator-as-authority (S4) + the CE-3d byte-exact differential, in step with DC-EPOCH-19/20/21.