core: pluggable address decoder hook + standalone-coin authorize work-resend (BCH G3b served-coinbase)

[frstrtr]

Summary

Two src/core seams behind the BCH G3b served-coinbase result. Both are required so a miner CashAddr username resolves to a real payout output instead of a value-0 OP_RETURN coinbase (which BCHN rejects as bad-txns / BIP30). Bundled because both are needed for the same served-coinbase outcome; each seam has its own justification below.

This is an exceptional core/shared change (touches src/core/address_utils + src/core/stratum_server.cpp, the shared stratum path for every coin). NOT eligible for self-merge or auto-merge — surfaced to integrator for operator tap. Not requesting merge until the gates below are green.

Seam (a) — coin-registered address decoders (src/core/address_utils.{hpp,cpp})

address_to_script() now consults coin-registered fallback decoders after its built-in bech32/base58 formats fail. Core stays format-agnostic and holds no coin-specific address knowledge: a decoder is an opaque functor string -> scriptPubKey, returning {} to mean "not my format, try the next one". Thread-safe registry, additive, and inert until a coin calls register_address_decoder().

BCH supplies its CashAddr decoder in src/impl/bch/coin/cashaddr.hpp (header-only, included here). Pre/post evidence on the BCH harness: address_to_script("bchtest:q...") returned {} before the decoder was registered and the correct P2PKH scriptPubKey after — proving the fallback is clean and built-ins are untouched.

Seam (b) — authorize work-resend gate (src/core/stratum_server.cpp)

The post-authorize work-resend was gated on !merged_addresses_.empty(). Standalone parent coins (BCH/BTC — no merged chains) never populate merged_addresses_, so they kept mining the pre-authorize job, whose empty username_ produced an empty payout_script -> degenerate coinbase. The gate now fires when the now-known username_ or merged addrs can change the coinbase.

Single-fire (all coins): the change is a single guard around a single send_notify_work(true) — no second send added. The resend fires exactly once in every case.

Corrected new-firing scope (the prior "standalone-parent only" claim was incomplete — there are TWO new-firing classes):

1. Standalone parent (BCH/BTC, no merged chains) — username_ set, merged_addresses_ empty.
2. P2WSH/P2TR-primary LTC/DOGE (Cases 7-9) — a 32-byte witness program cannot reduce to a 20-byte hash160, so merged_addresses_ is not auto-derived even though username_ is set. Under the OLD merged-only gate these miners got no post-authorize resend and kept mining the pre-authorize degenerate value-0 coinbase. The widened gate now resends once (force_clean) so their coinbase carries the real payout — address_to_script() returns 00 20 <32B> (P2WSH) / 51 20 <32B> (P2TR). This is a latent-bug fix for taproot/segwit-script LTC/DOGE miners: a genuine, correct LTC/DOGE behavior change, not a no-op.

For a P2PKH/P2WPKH/P2SH merged primary, merged_addresses_ is populated on authorize, so the new disjunction is true for the same reason the old condition was — identical coinbase, behavior unchanged. ltc-doge-production-steward verified single-fire / no double-notify against the live serve path (handle_authorize, single send_notify_work(true), same coinbase build). Deterministic matrix coverage including P2WSH/P2TR LTC primaries lands in test/test_address_resolution.cpp Suite 8 (ResendGate.*, 7/7): asserts exactly-one-fire, merged-empty on the non-convertible path, the real 34-byte witness payout, and that the old merged-only gate would have stayed silent (regression witness).

Gates (pending — do not merge until green)

• Full rollup CLEAN (not UNSTABLE): Linux / ASan-UBSan / Windows + all 4 coin smokes (ltc/doge/dgb/dash) + bch
• Single-fire resend matrix incl. P2TR/P2WSH LTC + standalone-parent (test/test_address_resolution.cpp Suite 8 ResendGate.*, 7/7 PASS) + ltc-doge live serve-path sign-off (no double-notify, identical coinbase build)

Outstanding (tracked, NOT in this PR)

• BCH stratum serve-path decoder registration (rides BCH M5 pool-serve landing; the work_source.cpp call-site is not on master)
• Live won-block-lands-on-BCHN leg — bitaxe rig allocation (routed to vm-fleet)