nmc(v37): classify always-fire dual-path as BUCKET-1 (do not converge to core SSOT) (#500)

The NMC won-aux-block dispatcher fires the namecoind submitauxblock RPC leg
ALWAYS, even after a P2P win -- unlike core::broadcast_block_with_fallback,
which short-circuits on P2P success to avoid a double-broadcast. For a merge-
mined aux block the always-fire RPC leg is a per-coin consensus-path invariant
(duplicate aux submission is a harmless daemon rejection, never a silent drop).
Add an in-file v37 3-bucket classification banner marking this as BUCKET-1
(per-coin isolation, KEEP) so the v37 multichain migration -- or any agent
converging coins onto the core broadcast SSOT -- does not collapse NMC onto the
short-circuit SSOT and silently drop the aux leg. Comment-only; behaviour is
already KAT-locked by NmcAuxBlockBroadcast.DualPathAlwaysFiresFallback.

No consensus-value, build, or runtime change. nmc-fenced (src/impl/nmc only).

Co-authored-by: frstrtr <frstrtr@users.noreply.github.com>

Author: frstrtr

src/impl/nmc/coin/block_broadcast.hpp

@@ -74,6 +74,20 @@ struct AuxBlockBroadcast
 // client yet) -- each leg is guarded so the dispatcher never throws or
 // dereferences an unset sink. Never silent-drops a won block: with NEITHER sink
 // present it logs an ERROR (lost-subsidy scream) and returns any()==false.
+// --- v37 BUCKET CLASSIFICATION (3-bucket rule) -------------------------------
+// BUCKET 1 (per-coin isolation invariant) -- KEEP, do NOT standardize/converge.
+// This dispatcher deliberately does NOT delegate to
+// core::broadcast_block_with_fallback() (src/core/block_broadcast.hpp). That
+// core SSOT SHORT-CIRCUITS on a P2P win (if p2p_ok return true) to avoid
+// a double-broadcast -- correct for a single-chain won block (BTC delegates to
+// it). NMC is merge-mined: the external namecoind submitauxblock RPC leg is
+// fired ALWAYS, even after a P2P win (a duplicate aux submission is a harmless
+// daemon rejection, never a silent drop). Converging NMC onto the core short-
+// circuit SSOT would DROP the always-fire aux leg -- a consensus-path
+// regression. The always-fire property is KAT-locked by
+// NmcAuxBlockBroadcast.DualPathAlwaysFiresFallback. v37 multichain migration
+// MUST preserve this per-coin aux contract, not collapse it onto the SSOT.
+// -----------------------------------------------------------------------------
 inline AuxBlockBroadcast broadcast_won_aux_block(const P2pRelaySink& p2p_relay,
                                                  const AuxRpcSink&   aux_rpc,
                                                  const std::vector<unsigned char>& block_bytes,