dgb(phase-b): wire build_connection_coinbase to PPLNS->coinbase SSOT (producer seam)

[frstrtr]

Stacked on #330 (dgb/pplns-conn-coinbase-wire). Replaces the empty build_connection_coinbase() stub in DGBWorkSource with a delegation to build_connection_coinbase_from_pplns — the #330 SSOT that itself routes through the single compute_pplns_payout_split() the verifier (generate_share_transaction) uses. A new set_pplns_inputs_fn producer seam supplies the PPLNS weight map (walked from the ShareTracker), ref_hash, subsidy and donation script.

Design:

• Tracker walk lives in main_dgb (ShareTracker + CoinParams in scope); no sharechain logic leaks into the work source, mirroring set_best_share_hash_fn / set_mint_share_fn.
• FENCED: while the seam is UNBOUND (or returns nullopt) the method is byte-identical to the pre-wire stub (empty job, no work pushed). Runtime behavior is unchanged until main_dgb binds the producer (follow-on slice).
• Byte-identity to the verifier holds BY CONSTRUCTION — one payout implementation, not two to keep in sync.

Tests: 3 KATs (empty-until-wired; coinb1/coinb2 byte-identical to the SSOT called directly; nullopt->empty). dgb_work_source_test 26/26 green.

NEXT (follow-on): bind set_pplns_inputs_fn in main_dgb to a tracker-sourced lambda (get_v36_decayed_cumulative_weights / get_cumulative_weights under the tracker lock) + freeze merkle_branches/segwit onto the snapshot.

[frstrtr]

Superseded by #333 (consolidated to master). #329/#330/#331 merged into intermediate stacked bases that never reached master; #333 now carries the full PPLNS delegation end-state (step-1 walk + steps 2-3 verifier + per-connection coinbase + producer seam) as one auditable diff rebased on current master.