Split hush unity build: move ~14k lines of implementation from headers into hush_impl.cpp and hush_nSPV_impl.cpp

- Add HUSH_PRIVATE_IMPLEMENTATION guard to hush.h separating implementation from declarations
- Create hush_impl.cpp to compile all hush_*.h implementation as a separate translation unit
- Create hush_nSPV_impl.cpp to compile all hush_nSPV_*.h headers separately
- main.cpp now includes declarations only, no longer a unity build for hush code
- Add both new compilation units to Makefile.am

src/hush_nSPV_impl.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) 2016-2024 The Hush developers
+// Distributed under the GPLv3 software license, see the accompanying
+// file COPYING or https://www.gnu.org/licenses/gpl-3.0.en.html
+//
+// hush_nSPV_impl.cpp — Compilation unit for nSPV (simplified payment
+// verification) implementation headers.
+//
+// Previously, the nSPV implementation headers were #included directly
+// into main.cpp after hush.h. This file replaces that pattern by
+// compiling them as a separate object (hush_nSPV_impl.o).
+
+// Standard project headers needed by nSPV code
+#include "main.h"
+#include "notarizationdb.h"
+#include "rpc/server.h"
+#include "init.h"
+#include "txdb.h"
+#include "core_io.h"
+#include "key_io.h"
+#include "merkleblock.h"
+#include "script/sign.h"
+#include "cc/CCinclude.h"
+#include "wallet/asyncrpcoperation_sendmany.h"
+#include "wallet/asyncrpcoperation_shieldcoinbase.h"
+
+// Forward declare functions defined in other compilation units
+bool Getscriptaddress(char *destaddr,const CScript &scriptPubKey);
+
+// hush.h in declarations-only mode gives us hush function declarations
+// that nSPV code calls (dragon_rwnum, dragon_rwbignum, hush_notarized_height, etc.)
+#include "hush.h"
+
+// nSPV implementation headers — defines, structs, serialization, then fullnode/superlite/wallet
+#include "hush_nSPV_defs.h"
+#include "hush_nSPV.h"
+#include "hush_nSPV_fullnode.h"
+#include "hush_nSPV_superlite.h"
+#include "hush_nSPV_wallet.h"