DC-NODE-20

Local selected durable chain forge-base authority (rung-1 single-producer). In a declared rung-1 single-producer venue, after Ade self-admits a valid forged block through pump_block (DC-NODE-12) onto its local durable ChainDB spine, the next forge base is Ade's LOCAL SELECTED DURABLE TIP -- the head of its own admitted ChainDB spine (ChainDb::tip) -- NOT followed_peer_tip and NOT an operator adoption certificate. This RETIRES, as forge authority, the upstream durable_servable_tip == followed_peer_tip re-check (which fired NoTipAvailable on every tick after Ade self-admitted a block the non-producing relay does not re-announce) and the DC-NODE-18 cert-promotion-into-extend mechanism. The local-tip forge base engages ONLY while ALL hold; ANY failure FAILS CLOSED (no silent fallback to followed_peer_tip or the cert): (1) VenueRole::SingleProducer; (2) NO competing block has been observed on the canonical peer receive stream since initial catch-up / self-admit -- an OBSERVED-FEED fence, NOT fork-choice; if observed, fail closed, do NOT resolve (that is rung 2); (3) the relay is non-producing; (4) the block was admitted through pump_block (DC-NODE-05 stays the SOLE durable admit authority -- DC-NODE-20 only READS the tip pump_block produced; it advances no tip); (5) the ChainDB spine is contiguous and servable; (6) no fork-choice decision is required -- in rung 1 mechanically DERIVED from (2): no competing candidate observed => the local spine head is the degenerate selected tip; a competing candidate => fork-choice required => DC-NODE-20 disabled. The FirstOwnBlockServed cert-wait intermediate is FOLDED OUT of the authority path: the transition is DIRECT -- (CaughtUpToPeerTip + self-admit valid own block via pump_block + the fence) => SingleProducerExtendOwnDurableSpine{current_tip = ChainDb::tip}, with NO cert read. DC-NODE-15 remains the INITIAL catch-up gate (durable == followed before the first own-forge); DC-NODE-20 supersedes ONLY the repeated post-self-admit durable == followed re-check. Relay adoption remains an EVIDENCE obligation for the transcript/bounty proof (DC-NODE-21), NOT a forge-loop precondition. In rung 1 'selected' is DEGENERATE (no competing candidate => local ChainDB head = selected tip); rung 2 MUST replace this with real fork-choice (DC-CONS-03, untouched here). Preserves DC-NODE-19 (continue-past-EOF in the extend state, now ENTERED via local self-admit not the cert) and T-REC-03/05 (the local-tip forge base derives from the local durable spine alone -- removing the RED cert/timing from the authority path makes the post-self-admit forge MORE deterministic / replay-equivalent). SCOPE: rung-1 single-producer only; follow-link keep-alive (OQ-KA), real fork-choice / multi-producer (rung 2), and preprod are OUT of scope.