dgb(phase-b): wire --redistribute selector into the won-block payout path (follow-up to #307)

[frstrtr]

Follow-up slice to #307 (Redistribute V2 policy port), per integrator direction: arg parsing + selector into the DGB won-block coinbase pubkey path. Stacked on #307 (consumes src/impl/dgb/redistribute.hpp); retarget to master once #307 lands.

What

• main_dgb: parse --redistribute SPEC (opt-in), build a configured dgb::Redistributor, bind DGBWorkSource::set_fallback_payout_fn (new producer seam, parallel to set_mint_share_fn). Threaded through run_node().
• work_source: FallbackPayoutFn seam consumed at the ShareAccept mint branch only when address_to_script(username) yields empty (empty/broken stratum creds). Unbound => byte-identical to before (default run unchanged).
• donate identity resolves byte-for-byte to the canonical V36 combined-donation P2SH. scriptPubKey is built from the raw 20-byte hash160 — explicitly NOT uint160::GetHex(), which reverses byte order and would stamp a reversed hash (caught by KAT during bring-up).
• redistribute_test: +3 KAT (arg-spec -> hybrid weights; donate -> byte-identical combined-donation P2SH; fee-without-operator -> fail-safe null). 8/8 green; c2pool-dgb links + --selftest OK.

Safety

• Node-local, consensus-safe: sharechain validation/codec/PPLNS untouched — only the locally-stamped pubkey on shares this node mints.
• Fail-safe: empty fallback left empty; unconfigured (null-hash) identity yields no script — never a burn output to the all-zero hash.
• Per-coin isolation holds: src/impl/dgb/ + src/c2pool/main_dgb.cpp only; no ltc/doge/btc/shared-base touch.
• Bucket-2 (cross-coin standardization). Payout-adjacent -> surface-for-tap, no self-merge.

Deferred (next slice)

• pplns/boost/fee script derivation through the generic pick() path (raw-byte conversion already in place; needs identity plumbing + live tracker-walk threading review). Operator payout identity for fee not yet plumbed -> fail-safe null today.