DC-EPOCH-15

Forecast horizon <=> durable N+1 authority promotion. The relay loop's EraSchedule forecast horizon extends past an epoch boundary N->N+1 IF AND ONLY IF the ActiveEpochAuthority has durably promoted the N+1 view (DC-EPOCH-14). (a) NO PRE-EXTENSION: the horizon never reaches into an epoch whose view is not promoted -- a header at an unpromoted N+1 slot fails closed OutsideForecastRange before the leader view is consulted, never accepted on a stale horizon. (b) ATOMIC ORDERING: the extension is the second half of the SAME transition as the promotion -- promotion durable -> rebuild the immutable schedule including the N+1 summary -> atomically replace the relay-loop-owned schedule -> only THEN is post-boundary validation/forging permitted; no mutable shared reference may leave validation using the old horizon after the authority has promoted. (c) DERIVED, NEVER PERSISTED: the extended schedule is derived state. Its authoritative inputs are the durable activation record + the recovered promoted EpochConsensusView + the v4 sidecar venue geometry (DC-CINPUT-05) + the committed network profile. Warm-start RECONSTRUCTS the schedule deterministically from those inputs and requires byte-identical schedule identity to the pre-restart one; a second WAL field for the schedule is forbidden (redundant authority + a new mismatch class). (d) NO AMBIENT INPUT: no flag, no wall-clock, no peer/CLI datum influences the extension (DC-EPOCH-13). Warm-start in BOTH states (before and after promotion) reconstructs a forecast boundary that exactly matches the live one.