build(win): make the mingw cross-compile link and find librustzcash

Three things broke the x86_64-w64-mingw32 build; two are real and fixed
here (the third was a stale-object contamination from building Linux and
Windows in the same tree, resolved by a clean rebuild — not a code fix).

1. Single-pass mingw ld could not resolve the cross-references DragonX
   added between the internal static archives (libbitcoin_util/common
   objects pulling in UniValue; util<->common mutual deps). GNU ld on
   Linux re-scans archives so it never surfaced; ld64 on macOS rejects
   the grouping flag outright. Bracket each binary's _LDADD in
   -Wl,--start-group/--end-group, delivered via AC_SUBST(LINK_GROUP_*)
   so automake does not reject the linker flag inside _LDADD, and left
   empty on every non-Windows target.

2. The Rust build emits the mingw archive as rustzcash.lib, but the
   link line asks for -lrustzcash, i.e. librustzcash.a. Normalize the
   staged filename to librustzcash.a for every host (a no-op on
   Linux/macOS, where the basename was already librustzcash.a).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Makefile.am

@@ -489,7 +489,7 @@ if TARGET_WINDOWS
 dragonxd_SOURCES += bitcoind-res.rc
 endif
 
-dragonxd_LDADD = \
+dragonxd_LDADD = $(LINK_GROUP_START) \
   $(LIBBITCOIN_SERVER) \
   $(LIBCURL) \
   $(LIBBITCOIN_COMMON) \
@@ -527,6 +527,8 @@ if TARGET_LINUX
 dragonxd_LDADD += libcc.so $(LIBSECP256K1)
 endif
 
+dragonxd_LDADD += $(LINK_GROUP_END)
+
 # [+] Decker: use static linking for libstdc++.6.dylib, libgomp.1.dylib, libgcc_s.1.dylib
 if TARGET_DARWIN
 dragonxd_LDFLAGS += -static-libgcc
@@ -553,7 +555,7 @@ if TARGET_WINDOWS
 dragonx_cli_SOURCES += bitcoin-cli-res.rc
 endif
 
-dragonx_cli_LDADD = \
+dragonx_cli_LDADD = $(LINK_GROUP_START) \
 	$(LIBBITCOIN_CLI) \
 	$(LIBUNIVALUE) \
 	$(LIBBITCOIN_UTIL) \
@@ -566,8 +568,10 @@ dragonx_cli_LDADD = \
 	$(LIBBITCOIN_CRYPTO) \
 	$(LIBZCASH_LIBS)
 
+dragonx_cli_LDADD += $(LINK_GROUP_END)
+
 if ENABLE_WALLET
-wallet_utility_LDADD = \
+wallet_utility_LDADD = $(LINK_GROUP_START) \
 	libbitcoin_wallet.a \
 	$(LIBBITCOIN_COMMON) \
 	$(LIBBITCOIN_CRYPTO) \
@@ -579,6 +583,7 @@ wallet_utility_LDADD = \
 	$(LIBZCASH) \
 	$(LIBZCASH_LIBS)\
     $(LIBRANDOMX)
+wallet_utility_LDADD += $(LINK_GROUP_END)
 endif
 
 # hush-tx binary #
@@ -591,7 +596,7 @@ if TARGET_WINDOWS
 dragonx_tx_SOURCES += bitcoin-tx-res.rc
 endif
 
-dragonx_tx_LDADD = \
+dragonx_tx_LDADD = $(LINK_GROUP_START) \
   $(LIBUNIVALUE) \
   $(LIBBITCOIN_COMMON) \
   $(LIBBITCOIN_UTIL) \
@@ -602,7 +607,7 @@ dragonx_tx_LDADD = \
   $(LIBZCASH_LIBS) \
   $(LIBRANDOMX)
 
-dragonx_tx_LDADD += $(BOOST_LIBS) $(CRYPTO_LIBS)
+dragonx_tx_LDADD += $(BOOST_LIBS) $(CRYPTO_LIBS) $(LINK_GROUP_END)
 
 # Zcash Protocol Primitives
 libzcash_a_SOURCES = \